I usually use a few things to decide if I think a game is worth my money:
1) Hype/Word of Mouth: I keep hearing how amazing Bioshock is going to be. It's been in the spotlight for a while now, and I kind of know what to expect from the game. If one of my friends tells me I HAVE to try a certain game, I will either borrow it or look around, and see what other people and reviews have to say.
2) Style: Is this game the type of game I'm into? I like action games, and if the game is turn-based, I most likely won't like it. This is something to consider.
3) Reviews: Important thing here is that the final score isn't as important as the information written in the review. Are the controls all messed up? Is the game pretty? What is multiplayer like? What is the story? etc . . .
I never rely on any single one of these sources. I like to get a feel of what MANY people thought about the game. I will generally read a couple of the highest rated reviews, and a couple low rated reviews. It gives you a lot more perspective.
