Just wanted to give it the praise it deserves. Transducers this good should never stay in obscurity. Long life to this living legend!

For me, they are the best ever. Maybe someday i'll listen to something 'better'? Well, we'll see.
 
PD: The Highs can bother some people with their 2k and 8-9k peaks. For me it's fine, but it's because I agree with Etymotic in their intention of tuning these that way, but you can disagree with them. After all, this is subjective, and IME this is the best headphones I had the pleasure to listen and own. Thank you very much dear reader.

This is one my favourite headphones. They have fantastic build quality, are inexpensive, require little voltage to be driven properly, are small and discreet, and sounds better than some pricier headphones (IMHO). 
 
They also are completely non-fatiguing, and great all-rounders. They don't make bright and thin recordings sound even worse, but instead make them much more listenable. A complete win from Sennheiser. Bravo.

Initial impressions: They can't reproduce the tone of a bass guitar correctly, let alone a human voice. Estrange thing is that they aren't sibilant or anything, but some mysterious peak between upper-mids and treble make some instruments like cymbals and female voice sound too unnatural and away from realist.
 
Bass sometimes sound bloated, and doesn't have a proper texture and articulation. The sound is detailed in a strange way: they have the infamous E.A.R. from Sennheiser, which is probably the most unnatural presentation of soundstage in a headphone. They can put hidden special effects easily, but they lack the nuance and detail of a instrument performer. You can't hear the breath of an acoustic guitar performer, or the strings vibrating, also you can't hear the body of a cymbal, only a pathetic artificial crash.
 
They are efficient (110 dB a 1kHz 1vrms), so you won't need ultra power to make them get loud enough. For gaming and movie listening they are probably a good option, but there are much better headphones at the same price, with better isolation and sound. These headphones are very overrated and can't think of a way to use them with serious music listening, unless you want to make your music sound absolutely horrible.
 
EDIT After 100 hours: They went from mediocre to very good! The highs are now clearer, the separation is much better, the bass texture and extension is very, very good, much better than before. The mids now are much more natural, not congested and muddy like before. Vocals sound very clear and with not obvious flaws. Transients and speed are among the best i've heard, not too far from spectacular. It sounds like a completely new headphone! Remember to give them time to 'open-up' or 'burn-in', as some like to call it. I TOTALLY believe in burn-in in Dynamic transducers, YMMV. Right now I like to call them 'The Closed HD600'. A very good headphone, but not perfect, damn good yes it is IMHO.