DISCLAIMER: I am KZ hater...I very much dislike the KZ house sound with its treble boosted to 11, its lackluster hollow mids and boomy bass. My extreme negativity towards KZ got me banned from the KZ forum on head-fi. They know me as the Marshall Mode guy. I much prefer darker, more forgiving earphones.

Build & Comfort:

The ES4 is made out of a hard plastic shell and comes with a removable cable. These earphones feel pretty solid, but with such a hollow and large casing I would be afraid to step on them. Comfort is rather good, I especially like the ergonomic shape and the lighter weight of the plastic casing compared to metal earphones that often fall out.

Sound:

The overall sound is very reminiscent of the Koss Porta Pro. You get elevated bass and treble, but the mids aren't too recessed. The soundstage is quite narrow and instrument separation won't blow you away. In fact, the ES4 gets kinda confused on busier tracks. The bass is definitely boomy and bleeds into the midrange. But it's a fun kind of bass and forgivable considering the rock-bottom price. The mids are so-so. On some tracks I feel like the vocals are singing through a paper cup but it's very subtle. On other tracks it's less noticeable. The highs are in my opinion the ES4's weakest point. It's kind of tinny but nowhere near as bad as other KZ earphones with lethal amounts of treble like the ZS5 (V2). In rock tunes for example, the cymbal crashes take over the entire song and that gets annoying. There's definitely an emphasis on the "T"s and the "S"s, leading to sibilant vocals. Let's be honest, the treble ain't smooth, it's quite grainy. This is when you realize that you paid less than $20 for these earphones. At the very least the ES4 provides an enjoyable and fun listening experience (especially with electronic music and prog rock) without stupid murderous amounts of treble (coming from KZ...). And none of the flaws are immediate dealbreakers, just a reflection of the rock-bottom price. If you want cleaner bass, more forward vocals and buttery smooth treble you'll have to pay more. How much exactly? $50~$100 probably... I am looking at the KEF M100 specifically...

I am giving this AKG K92 headphone a generous 4/5 because the price is rather low at $59 so the flaws are easily forgiven.

Comfort:

These headphones are very comfortable thanks to the humongous pads, their light weight and low clamping force. However, people with larger heads will complain about the elastic headband applying pressure over their skull. I am hoping that the elastic band will loosen up over time.

Build:

Right off the bat, I need to mention the rather long non-removable cable. It's rather flexible and solid feeling. Personally, I am not too worried about it since these headphones are just meant to be used at home and shouldn't be plugged/unplugged very often. Otherwise, I like to see that the headband is made out of springy steel and that the earcups are made out of solid plastic. This headphone feels solid due to its basic construction without any obvious flaws.

Sound:

Mids

Right off the bat, the mids feel kinda muffled and hollow on some tracks. Acoustic tracks with intimate vocals will lack vocal energy. Alternative tracks with loud guitars and quieter vocals will sound muffled. If you're a Smashing Pumpkins fan, these headphones are not for you. If you listen to a lot of Jack Johnson and Norah Jones, these headphones are not for you. This one flaw should steer many audiophiles away. But considering the price, most audiophiles would spend more money on "better" sounding headphones anyway.

Bass

This is where the AKG K92 really shines. The bass is punchy (without being muddy or boomy) and will make you groove to the music. The K92 really delivers with EDM music. It's not deep or fast bass like you would expect on far more expensive audiophile headphones, it's fun bass.

Highs

The highs are definitely boosted, without sounding shrill or fatiguing. It definitely makes smaller sounds "pop" and makes you believe that the K92 is more resolving than it actually is. There is some grain, but not as much as most headphones in the price range. It can be slightly fatiguing on some guitar tracks, but you shouldn't buy the K92 for rock/acoustic music anyway. I want to praise the K92 for not having fatiguing treble like the Philips SHP9500 or the god-awful Superlux HD 681. The K92 is truly an EDM headphone and the boosted treble reinforces that.

Overall sound

The overall sound of the K92 is that of a V-shaped headphone, not unlike the Beats Solo 3 which sounds rather pleasing, especially with modern pop music and EDM. The soundstage is in your head and the imaging is nothing remarkable. The K92 is not the most versatile headphone (it's not versatile at all LOL), and it's definitely not "Studio Monitor" material. But it doesn't have to be! The K92 is only 59$ and it does a wonderful job at putting a smile on my face when listening to EDM/Hip-Hop/Pop. Unlike other V-shape headphones, it doesn't sound overly fatiguing with piercing treble or boomy bass. I could see many people liking the K92 over more versatile and resolving headphones like the Philips SHP9500 simply because it's a joy to use with modern music.

Objectively speaking, the K92 should get a 2.5/5 rating due to its disappointing mids. But that's comparing the AKG K92 to headphones that cost twice as much like the Sennheiser 558 and don't sound as fun with EDM music. I give the K92 a 4/5 because the fun/price ratio is very high and the flaws are easily forgiven given the price. It's $59, what do you expect!?

I bought these KSC75 clips because I lost my modded Koss UR55 and I read oodles of praise for them online. At first I was skeptical because they are very flimsy looking and they are often compared to the Koss PortaPro which sounds muddy in my opinion. So I tried them on and I was immediately impressed by the overall balance and airy sound. Everything sounds so natural and forward, and it's really easy to separate vocals and instruments. I really like these with pop and rock music. The KSC75 crush the PortaPro. However, I must say that the treble feels grainy at times, especially with shimmering electric guitars (typical in post-rock). Another complaint would be that they look super dorky on your head, especially next to the PortaPro which is very stylish. 
 
I think that the KSC75 would almost better the Grado Sr60i, if it weren't for that grainy treble. They are that good. 

Well, I created an account just to review these. So here it goes:
 
I was looking for an inexpensive (<50$) portable pair after my incipio f38 and my JVC s400 broke down (these are ridiculously good headphones for the money by the way). I got these after I read tdockweiler's review and mod topic. Out of the box, these headphones were pretty mediocre next to the similarly priced headphones I mentioned above. They sounded muffled with muddy bass. So I performed the mod successfully. Removing the pads was easy thanks to my long fingernails (yuck), but the cheap screws made it a challenge to put the headphones back together. Furthermore, I can't seem to fully remove the cloth obstructing the driver, but that's a minor issue.
 
Well, I have to say that they sound a lot like my Grado sr60...with less bass. And that's a good thing because the Grado is not as portable, costs more and has an annoying cable. The treble is silky smooth (that's the selling point here because most headphones have sparkly/sibilant treble which makes me cringe), the mids are clear, the bass is very clean but a little shy for my taste. The soundstage is incredible and the overall level of detail is amazing for sub 50$ headphones. The vocal reproduction is outstanding for the price. It's superior in my opinion to the headphones mentioned above, but you sacrifice isolation and bass. I find them comfy, but the foam pads do make my ears sweat after 1 h of use. I think that the amount of clamping force is just right. The cable is fantastic: it's short (I don't see the point of long non-removable cables when you can buy an extension cord for a few bucks), it's tough and light.
 
EDIT: I had this headphone for weeks now and I have to say that I am still blown away by the overall performance. The bass seemed to open up after some burn-in and it is now on par with the Grado Sr60. The killer features are really the soundstage, vocal reproduction and speed. No other headphone in it's price range can compete with the Koss UR55 on these features, not even the Superlux hd 681 EVO. However, there are 4 drawbacks: 
- Noise isolation is non-existent.
- The foam ear-pads make my ears warm after 20 minutes.
- As with the Grado sr60, I wish that the bass would extend deeper. I feel like I am getting the mid-bass but not the deep-bass. Few headphones in the sub 50$ range reproduce deep bass anyway, so it's really a minor issue.
- They are ugly when the grill is removed.