I tend to head over to GameFAQs and looked up the game forums. Based on the questions asked and reviews given, can roughly get an idea how the game is like. Of course, YouTube makes it much easier to have a feel too.

Metacritic is good for a general idea on how well the game is received by the game reviewers.

In the end, sometimes it still depends on myself though. GOW series dominated a lot of score charts and gotten more than enough fans, but I have yet to actually ever play it at all. Instead of getting GOW for my PS3, I ended up getting Disgaea 3 (loving it madly) which is far less valued so far.