It's a combination for me. I read reviews because games are expensive now. Back in the N64 days, games were cheap enough that if you didn't end up liking it, it was the same feeling as going to see a movie for two with snacks that you ended up not liking. Now that they're $50-60 a pop, I'm much more wary. Typically, the big games are a definite buy if they intereest me (ie I would have gotten Halo or Galaxy even if they got merely 'average' reviews). But if it's a game that merely has piqued my interest, I'll read the reviews and make my own opinions. A 70+ game like this is a purchase only if I'm very interested in it, and I rent it or play at a friend's house first.
Also, that's not counting the 'see a very cool-looking game on the shelf that I've never seen before but must have' syndrome that hits me once in a while 







