I'm a bit late on this one, but...
I have RWRP on my IT set, and I agonised over the decision.
If I had my time again, I would have chosen standard, but it's not enough to upset me. Yes RWRP cancels the hum in 2 and 4, but I hardly notice the difference (humwise) against 1, 3 and 5 - and I'm in a very hum-prone building.
While I agree with the guys who don't think there's much of a tonal difference, my perception is there's enough of one for me to have decided that my next set will not be RWRP. If I was stuck with it though (eg second hand) it wouldn't bother me at all - still sounds like I wanted. My perception is that there's slightly more transparency in 2 and 4 with a standard middle, but slightly more "quack" with RWRP.
EDIT: Bear in mind though, this is definitely personal opinion/perception :lol:. Especially the "more quack" bit. And that varying the height of the middle pup in relation to the bridge/neck (and the relative height of bass/treble on all the pups) probably has a bigger effect on the perceived amount of "quack" you get.