I'm much better at roleplaying when I'm DMing. You'd think it would be easier since I'd only have to concentrate on that one PC, but I'm so used to knowing everything about everything that when I'm not in control of the environment I can't roleplay as well. This probably also has something to do with the fact that I've only actually been a player in one game ever... It wasn't pretty at first, but I got better.
So maybe a 7/10 DMing (and roleplaying various NPCs) and a 4/10 Playing (roleplaying my one PC).
So I voted average.
Average. I find given the opportunity, I can be good, but I follow the flow of the gaming group I am playing with. Since most of the games are a bit combat intensive, it is very easy to forget about character personality, drive, etc.
I think I'm good. I know how not to metagame, I can do sensibly different personalities for different characters, and my characters are not "myself but better". I sometimes make suboptimal choices because that's what the character would do, and people at the table often tend to remember my character better than their own.
But the real problem for this thread is the definition of "good roleplaying", and I'm not touching that right now.
I think I'm utter crap at it, but then I've always hated anything i've written/created. Thinking about it logically, though, I'm probably 5-6/10 for both DM'ing and as a PC.
I think I'm pretty good, especially if I have a good roleplaying group to play off. At a NASCRAG game during last year's GenCon, I had the DM laughing so hard she had to stop the game more than once and catch her breath.