I'm in Japan, and I bought a pair of ES55's for the Japanese going rate, which was around $85. I owned a pair of ES7's a while ago, but they went missing after I left them in a classroom for 10 or 15 minutes, so I don't have them to compare.
The ES55's are much smaller, lighter, and more comfortable that the ES7's, and they fit closer to my head, making them look less awkward. Also the cups don't press into my neck, like the ES7's did, while I'm not listening to them. My ears do start to hurt a bit after wearing them for extended periods, but the clamping force is less than the ES7's, so the pain is not as bad. Isolation is light, maybe less than the ES7, but fine enough for walking around.
As for the sound, the low end is warm and slightly prominent, but I think this is a good thing for portable headphones, as the low end is the first thing that you lose when you go outside; in my opinon, the ES55's are well balanced in portable use. I don't find anything very noteworthy about the midrange; maybe it just sounds "good enough". I think treble is the weakest aspect of these headphones. Treble sounds, like cymbal crashes and the top end of snares and drums, sound somewhat "pshh"-like sound instead of the detailed, sparkly sound I get from my RS-1's (Are the RS-1's spoiling me?). Anyway, this was really bad when I first got them, but I must say that it has improved considerably since then. The treble of the ES55's does sound better out of my Mint, but I don't expect people with a decent portable setup to be using headphones of this echelon, so it's a moot point.
Soundstaging and separation are fairly good in my opinion, maybe the two most noteworthy aspects of the ES55 sound; I can focus in on sounds and follow them with relative ease. On the other hand, I wouldn't call these cans detail kings. The overall sound signature feels fairly neutral (and not just in terms of frequency response) on the move, but definitely gets warm and fuzzy when you're in a quiet place inside, because of the bass quantity. If I recall correctly, the ES7's may have had less bass and a more colored (weird), plastic-y, but fun sound; it's hard to recall exactly, though.
I don't currently have any other headphones in the price range of the ES55's, but I'm fairly confident it doesn't blow any of them out of the water. However, the ES55's are a headphone that I can get into without thinking about it's flaws and get into the music, which is the best thing you can ask for a headphone and what makes me satisfied in keeping these. I'm not sure if I would pay another $45, or roughly 1.5x I paid, for them, though. For that price, you could probably buy some better, used headphone.
Notes:
I wrote this on a whim when I saw this thread come up as the one Portable Headphones thread featured on the Forums page, so I'll try to update it if I feel like these impressions are somehow totally irreflective of the ES55's. I also apologize for my wishy-washy language, but at the same time, I feel like it does reflect my feelings better than more polarizing words. And I have to reiterate that I don't have any other headphones with me besides a pair of recabled KSC-75's and RS-1's so take my opinion with a grain of salt.
For my portable sources, I use a 2G iPod Touch, which has a flat, dull sound, and a 1G Shuffle, which is more lively and punchy.