I guess the more a game caters to those people who post on message boards and such the more important the reviews are. For games like Dead Space, Red Dead Redemption, etc. review scores are very important.
On the other hand, as soon as a game caters to the mainstream ("casual") customer reviews don't matter anymore because these reviews don't even try to gain the interest of the mainstream customers. And the customers don't care about the reviews because the reviewers don't understand why people like certain games. Take Wii Party for instance. I bet people will love the game but reviewers will rate it a 6 or a 7 because "it depends on luck" or "the graphics suck" or "there are only 50 minigames and the motion controls don't feel very precise".
So basically reviews matter as long as the game in question is a game for people who like the same games the reviewers do. Of course, if someone made a magazine for the people who like the more mainstream games these reviews would matter for the mainstream games but currently such a magazine doesn't really exist 
