I don't buy many games anymore. Money is getting tight. When I did buy them more frequently, I would listen to what Yahtzee would say, read some on IGN and gamespot, glance through metacritic and also pay attention to the forums, trying to see what are people praising/damning about the game in question.
To cite two examples:
On the one hand, Yahtzee's praise of Driver: San Francisco peaked my curiosity about the game, and I almost purchased it during a steam sale; however, in reading some comments at screwattack and here on the forums of the escapist, some folks talked about a particularly frustrating level and some unforgiving aspects of the game. As I am not that good a gamer and do not find frustrating to be a selling point on a game, I decided to give it a pass.
On the other hand, Shamus Young's blog on Spec Ops: The Line, errant signal's video "essay" and some comments by the guys at RunButton podcast excited me enough about the game's story, design and commentary on shooters in general that I, who does not play any of the military shooters, took the plunge and bought the game.
It is rarely the numbers or the reviews themselves that will sway me. I look to an aggregate of comments and try to draw my conclusions from that.