Meh.
Personally I don't like them on account of content... I've always found the whole "hehe, I can run around killing hookers" mentality a little bit sad. That's mostly just the intellectual snobbery talking though. As a game goes it's... alright, hard to say really.
As a complete package the game is alreight. The last one I remember spending any time left was San Andreas (or possibly Vice City, who can tell the difference?) and I remember coming to the conclusion that it was a fantastic sandbox that unfortunately failed in every single game-play department.
Judged as a driving game the driving was bad. Judged as a shooting game the gunplay was bad. Judged as an RPG the role playing elements were bad and judged as a cinematic experience the story was pretty shit as well. Gameplay wise I could never find out what the fuss was about, and the game's supporters never seemed very sure of it themselves.
I'd go so far as to suggest that if you took away the giddy adolescent thrill of criminality from the games that people would completely cease to give a shit about the franchise.