I've never rented a game. It's been to my determent because I've certainly bought some stinkers and even paid full price on day-one for a couple (I'm looking at you, Two-Worlds). These days if it's a game I'm unsure of, I'll wait until I can pick up a cheap pre-owned copy.
If I do buy a game full price, I like to think I've learned enough now to pick ones that I know I'm going to enjoy.