It's ok, but some people put too much stock in it. I don't take it too seriously, but what I like about MetaCritic is I can read multiple opinions on a game in one place, I don't really pay attention to the average score too much. I get to read what people that loved the game thought, and what people that hate the game thought. It gives you more perspectives to say the least.
But the site does have some problems, take for example Eurogamer, they often have multiple regions for their reviews (such as Eurogamer portugal, Eurogamer Spain etc...) and each one of these reviews is pretty much the exact same in wording and score, but metacritic counts all these as seperate reviews. This tends to over inflate the score a bit, and some sites seem to boost the number higher.
But in the end I don't really care about that. I just use it for the multiple reviews so I can get more than one opinion on a game, some people take the site like it's the word of God but that's not right at all. People shouldn't put so much stock into it.