If it's a series I like, or a game I know is going to be good, I don't really do much research. But if I'm trying to decide if a game is one that I'll enjoy, reviews are just part of my research. I tend to trust the smaller, independent sites over the big corporate ones like IGN and Gamespot though. They get too many advertising dollars to be really trustworthy. I also don't just look at one site - I take a look at a wide variety of reviews, especially if there are some that are more positive and others that are negative.
But I also use gameplay videos/footage, feedback from internet forums/communities, and other methods to decide whether or not I'm interested in a game.