I suspect it sometimes because I get a game and it's garbage. I play it and I can't believe the reviewer didn't see some of the flaws or bugs in the game. It happens with Movie reviewers .
Of course. It happens with every product. Car magazines tend to highly rate the same brands over and over, and guess what, those car companies have ads in the same magazines. Same thing with consumer electronics. Most reviews outside of true consumers are a scam.
Overall I don't think it is the case. A number of reviewers have spoken out over this controversy and the fact is that overall there is no hard evidence of this occurring from what I can tell. Most high profile titles don't need positive reviews to sell, and if anything, reviewers come under fire for not giving a game ENOUGH praise. Google "Gears of War 3/Duke Nukem Forever/Uncharted 3 Review Controversy" and you'll see that nowadays reviewers seem to get crap for not giving perfect reviews for various games.
Overall I think you just have to consider the source (generally people who have to grind through these day in and day out) and consider a volume of reviews and not make a decision based on a single review.