For me it depends on the replay value of the game. Usually anything put out by Nintendo I always pick up; it never fails, I beat Metroid: Prime and Zelda: The Wind Waker a couple of times and put them aside... a few months later I started playing them again. Sports games I buy, but platformers that I can beat in a few days, I rent. The Movie Gallery here is nice, $3 for a week rental (on anything, not just games). There's close to equal shelf space for Xbox, GameCube, and PS2, so it works out well.
Another good thing is that if you're late it automatically rolls over, they charge you $3 again, but you get it for another week. So it's cheaper for me to play games I don't want to shell out $50 for. I still have a good selection of games (22 GameCube and 16 PS2 games), but why pay $50 for a game I'll only play for a few days?