Pros: Clear and very defined sound, good separation, good isolation, very light, decent comfort, plus a courtesy gold-coated 6.5mm adaptor.
Cons: Relatively fragile, lacks punch, low portability, somewhat unforgiving with bad recordings.
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.
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.