I only trust them when the game is either indie or not very well known. I'm still kinda mad that IGN gave DOOM a 7.8.
The main problem with these kinds of sites is that you rarely know who's reviewing what. If you subscribe to a game reviewer like Total Biscuit, or Jim Sterling, etc, you know the tastes of said reviewer, and you get an idea of what kind of games that person likes. IGN and others have a lot "reviewers" in their sleeve, so you don't get to know each one individually as much as you'd like.
So, do you trust them, or not?
Edit: An extra L on gaming, an the L is missing on professional. Not on purpose. Sorry.