My first impression usually comes from someone mentioning it. If it's a genre that I'm interested in, and it sounds like a good game, then I'll look into it further, often by checking with critics like Angry Joe, if possible. If I know any friends who've played it, that's even better because I can ask them what it is about the game that appeals to them and learn more about whether it seems like a game I would like.
Really, though, it's not possible to get a full idea of a game from someone else's opinions. You have to experience it for yourself. But you can get a first impression that is (usually) pretty helpful at deciding whether to get the game or not.
oh and if it's free i'll just go ahead and check it out without looking at reviews, because i might as well