I have owned the ES10's for a couple of months now and they have had at least 500 hours on them. In terms of sound quality, they have a smallish sound stage, probably due to their closed design. In terms of sound quality, I feel that they are in a different league from the likes of the ESW9 and even the ESW10. Treble and midrange on these cans are good, comparable in quality to the ESW9 and 10. Pretty smooth and relaxed for a portable headphone, but nothing to really be WOWed at especially if you compare them to the full size offerings of headphones such as the the big Sennheiser's HD600/650/800, but then again those are of a very different design and are an open design which I feel affects the sound characteristics.
The ES10 is very much about BASS. I love the bass on the ES10. In terms of absolute quantity of bass this headphone is a monster. It strikes very deep and hard but does lack control when used unamped, out of weaker sources such as the ipod and iphone. The bass presentation of these headphones, portray bass in distinct layers, and you can hear details in bass frequencies all the way till it is not heard but felt resonating inside your head. This headphone I feel has as much if not more bass quantity and reaches deeper than headphones such as the Sony XB700 ( the next uber bass monster) but while the Sony, lacks the midrange clarity and top end extension, the ES10's bass does not intrude into the midrange. And also has a pretty good treble extension.
All in all I feel that the ES10, is a great headphone to have is you love bass. If you mainly listen to electronic, industrial, dub, hip hop or other bass heavy music which is digitally mixed, this will be the "high end" portable headphone to have. There are many other higher end headphones that can make other types of music sound a lot better, but if your listening is mainly of the type I listed this headphone is very hard to beat. It is a headphone that is pretty niche and I know many an audiophile who will never even bat and eyelid at it, and comment about how colored a sound signature it has, or even how un-natural it sounds. This is not a headphone for eyes closed relaxed listening on the couch this is more a headphone for 500 bpm DOOMPH! psyching up for a long day at work on the the metro.