I am one. But i never have anyone to play with. I was thinking about actually buying the real PC version but i never got around to it. But one of these days i probably will buy GTA IV for PC.
Xbox wise, If i had someone to play with. I would be on a pretty good amount of the time.
San Andreas and Vice City are the only ones I really enjoy. Just that old school feel, it's great. Never really got hooked into GTA IV, shall be getting the new one however.