I find a group of reviews (particularly the top and bottom reviews as it were) can be useful to see if there's a common thread, but I always then factor in my own preferences and views.
For less known IP they can help get an idea about the game - for example Folklore reviews gave me a good feel for the game, plus it was interesting to see how broad the spread of scores were, a sure sign of a game that aroused very different reactions depending upon the player.
I guess when something is truly up (for example the control issue in Lair) the fact that every review calls out a major failing lets you know that unless you have a solid reason to invest you should be vrey wary.
The least useful reviews are those for big titles like Halo 3 or Bioshock - you know most reviews are going to be gushing while a few go against the flow and slam the game. For big titles I rely more on previews and general 'feel' on the internet.
I'm a patient buyer though so I always (well, Half Life aside but that's the only title I know I'm going to buy / enjoy) wait and see what the online reaction is as well before purchasing.
Try to be reasonable... its easier than you think...
