I'm no expert, but I have both the RE-2 and the RE-Zero and at this point I greatly prefer the sound of the RE-Zero.
Of my commuting headphones, the RE-2 is currently last on the commute list behind the RE-Zero, Radiopaq Classical and Yuin PK-3. I did like the RE-2 a lot better then the Meelectronics AI-M6 (which I no longer own).
My biggest problem with the RE-2 for commuting is isolation (or lack thereof). For home listening, I prefer an open sound like the RE-2. For commuting, I prefer a balanced sound because the mids help to block out a lot of distracting noise. I found the RE-2 unsuitable for a noisy commute on a full bus during rush hour because I could still hear loud conversations and in a few extreme cases, even the music other people blasting ear buds were listening to. The problem was compounded by the RE-2's mids which to me sound recessed and in the background. In fact, I even prefer my Yuin PK-3 ear buds for commuting over the RE-2.
As for the RE-Zero's, after only around 25+ hours of burn-in time, there is no doubt that these are my best sounding headphones. Very detailed and very balanced. The only negative I've noticed thus far, is the bulky adapter, which means that I will be forced to ditch the travelling case for my Sansa Fuze. Of course, the real test will be next week when I actually use my RE-Zero during my loud commute using real Sony hybrid tips (which I preferred over the stock tips).
Hope this helps.
Edited by murano - 8/12/10 at 10:38pm