DESIGN: Well, there is no way around it: the TD/80 is not pretty. The good thing is that it's not even trying, so it's not ugly either. Once I started to use and abuse it, it started to grow on me: I like its utilitarian Soviet approach, I really do. The thing looks like an industrial piece of hardware, and it's exactly that. Also, individual volume dials come in quite handy and help to give them their retro look. Not the best choice for your girlfriend, but definitely Macho cans to sport with a leather jacket and full beard.
 
CONSTRUCTION: For the price, U$S 40-50 in the early 2000's, build is quite impressive, particularly in late models. Every single piece of it looks like it can take the worst you can give to it, and then some. Forks? 100% metal. Pivots? same thing. Cord? thick, coiled, heavy, well relieved and with a meaty gold-coated J-plug. Housings? Lego-solid plastic. Headpiece? Leather-covered metal. It's no coincidence that they were a staple on record stores: these things have no weak spot and were made to last.
 
COMFORT: Well, the metal forks come at a price: the amount of pressure this thing can exert on your ears right out of the box is crushing. I haven't tried the AKG K81, but i'd bet my money they're just a wimpy bandana compared to the TD/80. Mind you, only the latest models have them, the old ones have plastic forks, which traded durability (and in my opinion, aesthetics) for comfort. The pivots are quite hard as well, so the thing has the ergonomics of a brick. Leave them on something to stretch helps, but still don't expect to wear this thing for more than hour and a half comfortably.
 
PORTABILITY: No folding whatsoever, and weight is almost 1/2kg. Let's say it's not the handiest thing around. On the bright side, it certainly is a toss-in-the-backpack type of can, provides decent isolation and has a coiled cord with a 3.5mm jack.
 
ISOLATION: With foam pads (early models), quite poor. With leatherette pads, fairly decent; bus-ride approved.
 
SOUND: Well, it all comes at this, isn't it. The TD/80 may be indestructible, but if it sounds like s**t, who cares? The good news is that it doesn't, it's actually quite good. Going along its overall design, the TD/80 has an all-out aggressive, in-your-face sound with a U signature.  As such, it's not a jack of all trades. First thing first: bass, bass, BASS. This is one for the bassheads. Lows are punchy, beefy and well defined. Mids are decent, with a noticeable dip on middle-highs. Highs are quite good, not shining but well defined, with some shining points now and then. Rock is benefited by its good treble, and sounds pretty well, albeit with a quite colored sound.  Not the best cans for vocal pop, due to its unemphasized mids. Apt for movies, though a more neutral sound would probably be more welcoming. I'd love to describe the sound better, but my experience is limited. In general, the sound has a somewhat murky, dark unhomeliness that might be a bummer for those in search for a clear, defined sound, but that I find quite appealing. I just can't stop to listen to Crystal Castles, Nine Inch Nails or Daft Punk on these things, while subtler and more delicate melodies are sometimes lackluster or odd-sounding (albeit quality is good to decent in 90% of the cases).
 
FURTHER NOTES:
- I do think they share the same drivers as the Porta Pros, though I haven't opened them up to see.
- As said before, late models offer many improvements over earlier models, particularly the pads.
- Modding probably would tame somewhat its aggresiveness.
- Produced at least between 1999 and 2003, currently discontinued, although I received a brand new one due to warranty (Koss still keeps a stock of it).
- In the same lines: Koss guarantee and customer service are simply AWESOME. I still can't believe all the trouble they took just for a 40 bucks old stereophones of some guy at Argentina.
- Tested with no amp whatsoever. Sound may probably improve with good amping. 
- Mind you, I'm not audiophile, just a n00b 'fier, so any input from an authorized listener will be much welcome.
- Hope you find this review useful or entertaining!
 
 
For the full story of how I got these cans in the first place: http://www.head-fi.org/t/649850/puzzling-experience-with-second-hand-koss-td-80-tough-luck-or-really-sh-tty-cans#post_9325013

Well, this is a product that gets a lot of praising and some bashing as well, so let's start by the obvious: they cost U$S 29, and can be found for U$S 20. They are really affordable. Audiophiles bashing them are missing the point: these are not cans for the audiophile, but for those who want to leave their as-issued earbuds in the shameful past and make a first step into a better, more defined sound. And in that way, the HD201 delivers splendidly.
 
The sound is the shining point here, very crisp and clear, with very good separation of instruments. Basses are gentle and quite controlled, with no emphasis whatsoever but quite clear and, in my non-basshead opinion, not lacking, definitely there. Mids are the shining point, with a very exact and pristine sound. Trebles are again well-defined. As such, they may be quite uninteresting for a Skrillex tour de force, but they are quite good for rock, pop and vocal jazz. Both male and female voices sound very faithfully and glossy. The hour of triumph for these cans, IMHO, is classical music and, in particular, piano-centered music. Most of what I heard through them was "cool, lovely, bright, pleasant, how agreeable", until I heard a rendition of Beethoven's Moonlight Sonata (mvt. 3) by Wilhelm Kampff and I was WOW!, each note shining like a perfect and crystalline snowflake. And, as in piano playing, tha't what the Senn HD201 are about: it's all about control. What it gets in, it gets out. As such, also, they are somewhat unforgiving with poor recordings or bad files. Their precise sound is also an excellent option for movies (maybe not for disaster movies) and gaming. Some say that it's a "boring" sound, I'd prefer to say it's an "exact" sound. I do think they're quite an improvement over all the obvious low-end Phillips/Panasonic/Sony options
 
Then again, these are cheap headphones, and as such they're cheaply made in 100% plastic and with an unremarkable, if sober, design. They don't look exactly cheap-o or flimsy, but they doesn't fool anyone about which is their price tag and should be treated gently. The bright side of their construction, if any, is that they are quite light for their size, with a well padded headband. Comfort is average, my limit was an hour and a half of comfortable wearing, after that they get too hot and sweaty. Isolation is decent. Portability is obviously not what designers had in mind, being cumbersome, unfoldable and with a long cord; yet the light weight and good isolation make them not that bad an option for a train or bus ride... if you treat them gently, have a roomy bag, and don't mind to look like a cosmonaut.
 
CONCLUSION:
If you are looking for a toss-in-the-bag portable which looks stylish and cool on the street, and/or for a booming-bass and "fun" sound to blast your Ipod's nu-metal or dubstep compilations, run like hell from them and go grab Skullcandy or iFrogz.

If you are looking for a sizable improvement of your stock gear on a shoestring budget, and your concern is sound fidelity, these are the path to follow into a brave new world of better sound. Soundwise, the Sennheiser HD201 are among the best value that can be asked for.
 
And, I found the gold-coated 6-5mm. adaptor just the complimentary mint of an already great product. What a steal.