this is my first set of customs, so I can't compare it to other customs. but comparing to a set of TF10, there is no comparison.
First, it's very comfortable. with TF10, my ears hurt after an hour or so of listening. but with RM, long listening is no longer an issue. I only put them down to take a break and rest my ears. of course, wearing them on/off is much more difficult, but I'm getting the hang of it.
Second, the ambient noise isolation is much better. I was surprised by the fit and the consequent isolation. it seals for good tight bass and block outside noise.
Third, it sounds awesome. the little details are brought forward. It's flat. no coloration whatsoever. I mix live music so this is perfect for me. i love flat sound: direct from the source without any coloration. if you're a bass head, then you should look elsewhere.
all in all, this is my new best earphones...
i waited six years to get a pair of customs... the wait was worth it.