I almost never buy games on release date, even if I've been following the game since it was first announced.
My main reasons are cost, and a recent trend in surprisingly bad games despite the previews (Examples: Alpha Protocol, Epic Mickey, Elemental: War of Magic). Someone already mentioned the perks of getting Game of The Year editions, which, along with Collectors editions are very good reasons to wait before spending money on games.
(Seriously, I'm highly tempted to get Dragon Age: Origins Ultimate Edition for PS3, for less than half of what I paid for PC version, with all the DLC I could never afford already included)