I've been a player of the GTA series since the first one. Even though the game is violent, it has a kind of uplift tone to it. The radio stations, dialogs, caracters, missions, scenarios, sound, etc., have a humourous touch. Everytime I play the last installement (San Andreas) I get a good laugh (ex: the "Maria" missions).
Another good thing about this series is that each new installment improves on many aspects. GTA IV is no exception, it even has multiplayer.
This is a game made (also) for grownups. It has violence, sex, humour and good playability. No wonder you see "normal" people in line waiting for it I would too, if I had a 360 or a PS3 :\