Not sure where else to post this, so I thought I'd resurrect this thread with a short comparison.
I just got my hands on the E500, and I do have to agree with what has been said in regards to the ES2 vs the E500. Now, it's far, far too early to post definitive opinions as to which one is better, but based on early listening, to my ears the ES2 is clearly superior. The E500 has a significant advantage in soundstage, provided one likes a more laid-back sound, as it does sound quite a bit laid-back next to the ES2. The latter is more forward and doesn't have the same soundstage width, though it has similar depth and height. Detail to me seems to be about the same for both, though the ES2 sounds fuller and detail is somewhat obscured by the foward mids and the huge bass (next to the E500, anyway). I suspect that the ES2 has slightly better inner detail retreival, but I'll have to listen much more to determine that for sure. Where the ES2 really pulls ahead, to my ear, and where the E500 really lacks, is in fullness. The E500 still has that same "canalphoney" sound signature that I've heard from the ER-4P and the E4c. Admittedly, it is by far the best iteration of that sound signature that I've come across, but next to it, the ES2 sounds like a full-size headphone. It's simply fuller, more present, more dynamic, more, well, "there." Instruments seem to occupy much more space than they do with the E500, which sounds a bit thin by comparison, and they seem to have much more solidity. Also, the ES2 seems to be a bit more refined especially in the upper mid and lower treble, where the E500 is fairly steely (unamped). On the plus side for the E500, it does have a very good midrange in it's own right, very transparent and very reminiscent of the E4c, and it's bass is very tight, deep, and not lacking in quantity at all, as it was with the ER-4P and E4c.
Well, that's about it for now. I'll have to listen much more. I like the E500 a lot, though it's from the ER-4P/E4c school of sound (detail first), which I like less than the UM2/E5c/ES2 school of sound (fullness first). But, it is the best universal-fit that I've come across, that's for sure.