Yes, it happens, I know, but it is not Metacritic's fault.
As a rule, it always good to read as many reviews you can. Also, play the demo, see videos and such things. Metacritic is a nice tool, but it must not be the end all tool. Also, go see the players score. Usually the players are people more like you than the critics, so their scores tend to be different. It also forces sites to be not just a review site, but to do more things like interviews and discussions to generate traffic.
In the end, there is no perfect system to choose games, except for the hands-on system. But when in doubt, the Metacritic is in fact a good place to start. The more well informed you got the better chance of buying great games and avoid the sucking ones you have.
PS: Participate on Fightoid! Details at http://www.destructoid.com/blogs/ManWithNoName/friday-fightoid-delayed--204024.phtml. Nobody have took an instance for Sonic, so help the blue hedgehog to kick Mario's ass. Also participate on TOP 7 Game Scenarios. Details at http://www.destructoid.com/blogs/ManWithNoName/top-7-things-204091.phtml