OK so far I think they are a step up from the RE0s straight out of my Fuze. RE0s probably do better with an amp, but that's for others to try as I no longer have the RE0s or an amp. Overall the reviews already posted are accurate in describing the sound of the RE252s, but I noticed a couple of other things myself.
First the bass seems to go deeper when compared to the RE0s, thought the difference might be slight. Second i enjoy the separation/soundstage on these a lot more compared to the RE0s. I can actually separate the different instruments in my head without one interfering with the other.
Unfortunately other flaws come up with the RE252s. The build quality on these isn't as great because you have to make sure you don't rip the gel attachment on the earphones and as mentioned earlier the filter is somewhat sensitive. But worst of all is the design of the earphones themselves. The design limits you to use them straight, which won't work for everyone and you can't insert these deeply into the ears. After experimenting I have found other ways of placing them in your ears than the one you are limit to by it's design, that work a lot better but look very silly because of the gel part sticks out. Therefore because you can't really experiment with the fit, getting good isolation from these is tough. But sometimes even with a bad fit, I'm generally satisfied with the isolation (thought I can't compare it directly with another iem). The plus side on this is that they are easy to place in that you don't insert them very deeply, but getting the gel part of the earphone to be place properly prolongs the insertion.
Overall, Even though I like the sound, I really hope they change that horrible design.