Personally, I don't think I'll ever get tired of playing the GTA trilogy. I stick with San An 'cause it's the biggest and has the most variety but I still play VC if I'm feelin' in an 80's mood.
I think I've been playing San An everyday for a year (man I'm soooo addicted) and I still have only got to 80%. Then again I kinda left off tying up the loose ends as I got absorbed in generally farting about and doing stupid things, like seeing how far I could jump off the top of chilead (980ft in an Infernus w. nitro, haven't tried it with the tank yet tho ) or collecting police vehicles (including the police maverick WHICH HAS NO DAMN GUN )
Anyway, yeah I love all the GTA's and don't think I'll ever get tired of playing them (except when APB comes out)