I typically ignore Metacritic until it dips below 60 points, at which point I have to ask, "Am I going to risk it?" And that was how I managed not to fall for The War Z's name chance to Infestation: Survivor Stories.
Also why I ended up not getting Omerta: City of Gangsters, which is sad because I was considering preordering it because it looked fun.
Metacritic averages are great at indicating things like how buggy a game is, and how memorable it is. So that's how I use them. You don't crack 90 unless you execute really well on all the basics; you don't reach the low 60s unless you have fundamental flaws with your game. The fact that a game has a high or low metacritic doesn't say that much about whether it is in a genre any individual player will be interested in, whether it had interesting ideas, what its particular strengths and weaknesses are... but for that, you can watch a youtube video or two; maybe *gasp* read a review.
For the curious... Metacritic stats!
Metacritic has (currently) 3020 PC games with scores, ranging from 96 (Half-Life, HL2, Bioshock, and... Out of the Park Baseball 2007!

A score of 89 or above puts a game solidly in the top 5% (there are actually exactly 100 games with scores of 90 or above right now). A score of 80 or above puts a game in the top 25%. 72+ puts a game in the top 50%. 62+ puts a game in the top 75% (or, not in the bottom quartile). Below 45 is the bottom 5%.