There is an element of truth to the claim that fanboys will score games for their favourite platform higher and games for another platform lower but I think that (in general) these numbers tend to balance out ... I wouldn't use the user score to argue one game was better than another, but if there is a large disparity between the user and the reviewer's scores it does indicate that there is something that is up.
One of the big problems I have seen over the years of reading videogame reviews is that reviewers tend to play games in a drastically different way to how most gamers will experience that same game. Some games (like Halo 3) have somewhat short and disapointing single player campaigns and get high scores primarily because of their online multiplayer, and a majority of gamers still don't play videogames online; some games (like Mario Party 3) are built around local multiplayer settings and are very enjoyable in them, and some reviewers only play the game as a single player game and then complain that it doesn't have online multiplayer; and then there are games (like The Conduit) which generally don't stand out when compared against a dozend games that the average person interested in the game will have never played.
