First Impression
 
When I first listened to these headphones, my initial two thoughts in this order were:

1. Wow these sound pretty fast and also fairly even

- 15 seconds later -

2. Wow something is wrong with the upper mids
 
 
Packaging
who cares
 
Build Quality/Aesthetics
These headphones feel very well built.  I can't speak to their longevity or durability, but they are nice to hold, feel sturdy, the movements are tensioned well and feel solid, there's no creaking of cheap plastic.  The finish feels durable and well made.  One of the better headphones out there IMO for it's apparent build quality and finish. 
 
They look very nice when sitting on your desk, and in pictures.  A good design, but they are one of those headphones whose looks don't entirely translate to looking good when worn.  They have all the right shapes and dimensions relative to each of the parts, but the whole thing is just too big to look very good.  For their size, I'd say they don't look too bad as they are slim, so they don't stick out horizontally very much and give you the alien mind probe look that grados and many closed headphones do.  The headband is slim too since it comes together at the top of the cups, so it doesn't have that gigantic and unnatractive arc that the older AKG's have like the K240.  The headband hugs the sides of your head well.  Still, the cups are huge even on my big noggin.
 
Comfort
Very good except that the earpads just aren't thick/deep enough.  Huge design flaw IMO because everything else was done very well for an extremely comfortable headphone.  The cups swivel in a really nice way since they have a good amount of friction in the movement, so you can place them at the right angle for your head, and they stay there.  Headband is comfy and is the right shape.  But the earpads aren't thick enough to keep the grill over the drivers from hitting your ears.  With such huge, soft earpads this is a pretty big fail IMO.  I tried putting the foam donut shaped inserts that Hifiman has in their earpads into the K550 earpads, increasing the thickness, and it had no effect on the sound and made them supremely comfortable by simply increasing pad thickness.  So as long as you don't have huge ears they should be fine.  Or if you don't mind a little pressure from the grills.  it doesn't hurt too bad on my big ears, but it just would have been so easy to avoid this situation.  I get the feeling they didn't do much real world testing on these and got carried away with computer modeling. 
 
 
SOUND QUALITY
 
They overall have a flat, balanced sound from bass to midrange with good tonality, and with a treble presence that is in balance with that part of the spectrum, which is very rare for headphones.  But there is a serious problem spot in the upper mids that ruins this headphone.  A resonant peak in a small part of the upper mids.  I would describe their overall sound as a slight rainbow shape FR, though leaning upwards towards the upper mids for enhanced sense of clarity.  They also can sound kind of fast for a closed dynamic.  Not ortho or stat fast by any means, and not Grado fast, but maybe 6/10 for speed.  Not bad. 
 
Bass
Good not great.  It extends down plenty low, it seems flat, there's no apparent midbass hump, bass does not bleed into the midrange or vocals in the slightest but is also well integrated with the midrange.  There's no "bass + mids"  or subwoofer effect where the midrange and bass feel separated by gaps in the spectrum.  They have a cohesive transition from bass to mids, likely from a fairly flat FR.  But bass is a bit low in quantity despite what the FR measurements suggest.  The bass is in the background on these phones, with emphasis on the midrange.  Not for bassheads at all.  Bass is not bad quality, but it sounds kind of dry and a bit soft, whereas better headphones have more definition and dynamics and detail in the bass.  Not bothersome, especially since it's in the background.  I'd describe the bass as being just enough to provide a foundation and not sound too lite, but too low to sound totally neutral- it keeps your attention on the midrange.  It gets out of the way so to speak, for better or worse.
 
Treble
I'd say the treble on these is very similar to the bass.  Good not great.  There's no emphasis in the cymbal range as far as I can tell.  If anything it sounds a bit recessed in the upper treble.  Nothing sticks out at you here or is glaringly bad.  But the treble is not that clean or precise sounding.  It is a bit rough/textured or papery sounding.  Like the bass, I'd say it's a backdrop for the mids.  Not a treblehead headphone either.  Yet despite this lack of apparent brightness, these phones have a strong sense of clarity about them.  I think this is what people like in this phone.  They don't sound dull or rolled off or warm at all.  Likely because of the slight lowering of bass volume, along with the emphasis in the upper midrange, the "clarity" range in the FR.
 
Midrange
Here's the problem.  Both the bass and treble on these are both presented as a backdrop for the midrange but there's a serious problem in this area, so these phones wind up failling short for bassheads, and treble heads, and then have issues in the midrange.  First the good part.
 
The heart of the midrange and the lower midrange is done pretty well.  Vocals have both the proper richness for a male voice like Tom Waits or Chis Isaak, which is surprisingly rare- they don't make vocals sound thin despite the lack of bass emphasis.  They also don't sound overly rich at all.  Chris Isaak's voice sounds very good.  Vocals also give the proper air and breadth to female vocals like Emmylou Harris or the Audiophile favorite Allision Krauss.  But sometimes vocals reach up into the problem spot and sound off. 
 
Upper Midrange Issues
There is a spot in the upper midrange that has a pretty bothersome resonant peak.  I kept wanting to call it "glare".  The upper midrange glare isn't the most peircing or painful I've heard in a headphone but it is very bothersome and might be described as severely "jarring".  It doesn't seem to cause me to reach for the volume knob as feverishly as some phones have, but it does keep me from turning them up and leaves me in a state of fear wondering when something is going to hit this problematic range and if it is going to hurt, since it seems to be right on the threshold.  Any instrument in that range sticks out way too loud, and looses proper tonality as the sound in that range gets mangled by resonance and takes on a plastic tonality. You can clearly hear the coloration being added to what should be the original signal.  This is partly because it's such a specific problem spot.  You can sometimes hear an instrument or voice go up in the range and suddenly jab at you by suddenly increasing in volume while taking on this plasticky resonant glare.  The decency of the lower part of the spectrum gives it a "surprise attack" quality which is what generates the fear and hesitancy when listening.  If there was an instrument that existed only in this range, I am doubtful you would be able to hear what instrument it was.  Some recordings play well with this problem spot and it's not grossly apparent because of the frequencies being excited by that recording and the particular balance of the recording (if the recording has a peak in that spot too, it's clearly going to be worse).  But even on these recordings, this spot gives the whole spectrum a cheap plastic headphone feel kind of layed over the sound, as if you are listening to some very good throw away headphones.  I kept thinking of them as the best airline headphones ever.  Much better extension, tonality, more flat, but still plastick-y. 
 
Soundstage
Overall good size for a closed headpone, has a spaciousness about it, but there's something strange about it as well.  It's almost like there are 2 soundstages happening at once, the lower part of the spectrum sounds more spacious to me than the upper mids/treble, which sounds more constricted.  This gives them a lack of coherency, or a kind of bad crossover effect, where the sound is a bit disjointed.  I think this is because the resonance artifact in the upper mids destroys soundstage by getting in the way of the "trick" the headphones are playing on your perception.  Soundstage is also destroyed by a "driver dildo" that was installed on the back of the driver creating a tiny chamber behind the driver rather than the comparatively larger size of the earcup.  I'm guessing it was placed there in an ill conceived attempt to control the bass, but I have modded a pair of these by removing this dongle, and correctly damping the cups, and the soundstage becomes more coherent. 
 
Conclusion
Good looking, sturdy closed headphone that is fairly comfortable and has a mostly well balanced, even sound except for a very problematic part of the upper mids that sticks out sorely and ruins them. However, this part may not bother everyone.  If it doesn't bother you ie if you're not sensitive to resonance or FR irregularities, and you are looking for a well balanced headphone erring on the side of being a bit basslite for the sake of clarity, and you want good soundstage these may be a good choice for you.  If you are not bothered by Grados you may not be bothered by this resonant peak.  however, Grados have much more natural tonality, for example vocals sound more life like.  I recently had an MS1 here and despite their colored signature they sound better than the K550 in every way except may soundstage width.  There's no plastic tonality in the MS1's signature. 
 
Here is a picture of the inside of the K550's cups.  The lack of damping here contributes to the resonant peak, but after spending time with my modded pair which has damped cups, I have found that it is also being caused by the driver itself.  Damping the cups helps, but doesn't fix the problem.  The small rubber thing in the middle of the light gray circle around the driver, is the "driver condom".  This seals around the driver and makes a small enclosure size behind it.  The light gray area are vents, though they seem to be closed off with solid plastic.
 

These headphones cost 1.95 including shipping from Amazon with a Prime account.  Link to Amazon  I got them to use the headband for a DIY fixup of a vintage phone.  But I figured I should write a review since they're so darn cheap.

Packaging

They come in a plastic bubble like thing.  The plastic seems relatively sturdy and protected the phones well.  It is also see through, which is nice so you can see all the glory of the product enclosed.  There was also some cardboard inside.  Regular cardboard, nothing special.  The graphics printed on it were a little dated and lacked the color and creative vision that might have grabbed me and made me more excited about the purchase. 

Build Quality

They feature a metal headband strap, which seems strong enough to do the job.  They're probably more durable than Beats and Bose from what I've read, I don't think they'd break even if you went Tyll Hertsens™ on them.  The plastic is cheap and makes very unpleasant high pitched "tip, tip" noises when you hit them with your fingernails, instead of the deeper "tap, tap" you would expect to hear from a quality plastic.  It was easy to break the plastic arm next to the metal headband to remove this metal strap for my project, so these aren't a good choice for your angry 5 year old kids.  Only if they know their manners.  The good thing is they cost less than 2 dollars.  

Looks

They look like super crappy headphones.  Which is kind of charming in that "I don't give a dam" grunge kind of way.  They'd go well with your sister's old beat up jeans and too small hoody.  And

Comfort

They're really light so that's nice, but the angle of the (fixed) earcup/baffle is straight vertical if not angled out the wrong way, so it only makes contact with my ears on the top of the baffle, and then the rest angles away from my ear, so that there is a gap of a finger's width at the bottom end (where the cable comes out).  This causes some unnecessary pressure on this contact point up top because the weight isn't well distributed.  It also negatively effects the sound.  

Sound

The most important thing when buying 2 dollar headphones is how they sound.  They sound exactly like you would expect 2 dollar headphones to sound although they do have some pleasant surprises.  They're not very harsh, there are a lot of very expensive audiophile headphones which exhibit much more harshness.  These are fairly smooth except for a peak of some sort in the upper midrange, but it's not bad.  The overall frequency response is a mountain shape (upside down V) which starts at 80hz or higher and climbs slowly upward until it peaks in the upper midrange and then starts it's decline, extending to 10khz or something probably.  Coby does not publish FR range specs on their website for obvious reasons.  I doubt they are flat in the upper midrange very long, most frequencies are quieter than the upper mids, they are not even flat throughout the whole vocal range.  Lower mids are recessed as they begin The Great Rolloff.  They also suffer from overall SQ because of the way they fit hanging away from my ears, this gap contributes to the upper midrange peak, lack of any bass whatsoever and lack of richness and warmth.  

They are also fairly resonant.  They kind of sound like the drivers were installed backwards and everything is bouncing off the curved plastic enclosure before getting to you.  

Treble

The treble sounds like it is being filtered through a thick paper wall.  It is extremely veiled, thick, really grainy, and for the most part nonexistent.  There is a complete lack of air and precision.  Cymbals sound like they are miniaturized and made of soft plastic or some kind of hard papery substance.  There's no simmer or sizzle at all.  I get the feeling that I may be missing entire instruments that exist in the treble regions.  Like bells and things.  

Midrange

The midrange also sounds like it is coming from behind something.  It is muffled and tends to get smeared into other instruments.  Listening to Led Zeppelin, his voice sounds like he's singing right next to the electric guitar sometimes so that the two sounds blend together a bit.  It has some characteristics of natural tone, it has some warmth and it's not terribly shrill with male vocals, though it tends to be a bit shrill with female vocals.  Lhasa De Sala's voice sounds a lot smaller and less rich than it should.  There is a sense that you're getting a little piece of the vocal range, with the top and bottom cut off, as if the vocalist is singing through a little slot in a wall that only lets the middle bit of vocal frequencies through.  But vocals do have a degree of naturalness to them, and they do manage to portray some of the breathy qualities of vocals.  I think the tone could be worse, it depends on the album.  Daniel Lanois' voice is portrayed much better than Lhasa's voice and is acceptable.  But overall it leaves a lot to be desired and after listening for an extended period of time, the amount of ringing and resonance in the midrange becomes quite fatiguing and I get a callaustraphobic feeling, like I want to get out of this tiny resonant chamber my head is trapped inside.  The resonant notes in the upper midrange in this song are much more echoey and resonant than they should be, as the Coby's add their own resonance to the reverb in the recording.  

Bass

The bass isn't part of the picture with these phones.  Bass guitar can hardly be heard.  If you had a recording of a hundred elephants farting in unison in a big gymnasium, I think through these headphones it would come across like a squirrel fart high up in a tree.  When I switch to my Yamaha orthos I realize that there are entire bass notes which do not exist on the Coby's.  It is able to portray a kind of a "puff puff" type sound with bass drum hits and some bass guitar notes, but it's more of a generic impact, and not discernible as a particular note or instrument.  It sounds like a baby banging on a small Fischer price version of the real thing.  Listening to Daniel Lanois' album "Shine", whose first song has a very deep bass note, the Coby's do actually capture the upper end of this note, which is surprising.  But it sounds strange, it has a different tone then it should, it again sounds like some kind of kids toy- a toy bass guitar that makes sounds when you push big plastic buttons where the strings should be.  There are further bass notes in this song where the Coby's capture the upper parts of certain bass notes, but the lower notes in the melody are non existent or fade as they go deeper.  

Separation and Imaging

Separation and imaging are decent for the price.  Some parts of the frequency range have some issues where notes get blended together, namely the heart of the midrange and lower midrange to the "bass", which is effectively all this headphone produces.  But there's a degree of separation and imaging in the upper midrange and treble.  I can clearly hear cymbals here and a vocalist somewhere else.  Imaging is better when there isn't too much going on at once.  When 2 notes occupy similar frequency ranges on the same channel (left/right) they get blended together and tend to occupy the same position in the soundscape, which can be so bad that is damages the integrity of each instrument.

Soundstage

Horrible.  It's not just inside my head, it's inside of the inside of my head.  Like I had headphones implanted an inch inside my head from each side, and these implanted headphones have tiny soundstage.  Some of the treble manages to get outside my ears a bit, but the vocals dive deep inside my eardrums with all their resonant fury and I desperately want to get them out.  

Sensitivity

Decent, not great.  I have the volume on my ipod at 50-70%.  for their size, they should be more sensitive I think.  

Conclusion

Save up for the KSC-75.  Don't buy these unless you want some crap earphones to toss around.  Not bad for 2 dollars to be honest.  But they have horrible extension on both ends, cutting off entire instruments in the bass and treble, with a slightly peaky, resonant midrange that is fairly smooth, but isn't entirely natural sounding.  A very narrow band of the midrange is the best, or shall I say, only functioning aspect of this headphone, which while resonant isn't terribly offensive.  But if a voice doesn't occupy this narrow band it's not a super fun time.  They have poor comfort and severe durability issues.  

I think their saving grace is the lack of harshness, which means I'd be comfortable with a kid using these, as I don't think they'd damage his/her ears.  I would be a bit afraid of the unnatural, resonant midrange damaging his/her brain, however, and I wouldn't want my kid's brain to be developing under these circumstances for fear they may not develop proper perception, or they might just go crazy altogether.  

My Macbook Pro laptop's built in speakers are better in every way including bass response, if that tells you anything.  They have a more natural midrange and more extension on both ends.