I've never tried in-game RPing before, although I find the concept intriquing enough, and I'm sure that in execution it could be a pretty interesting experience. Saying that though, I can't imagine I'd actually be any good at it since I tend to be pretty quiet whilst playing games because I'm concentrating on what's happening in the game, killing and dodging monsters etc etc lolol. 

I've always preferred play by post RPing in general though. I've never gotten into IM RPing either.