It's really all about preference. For example, let's say you're into the action genre games. Naturally you will tend to like these games more, even if the game is poorly reviewed. I believe reviewers try to be as objective as possible (well as least the good ones), and create a score that can be used as a general reference.
I tend to look for games in the genres I like, and then check the metascore. I compare the potential purchase's score with another's score of the same genre that i played and enjoyed