Reviews have a high effect on sales, but as has been said, are only one factor among many. Furthermore, they affect different games and gamers differently. Examining the factors that go into a game purchase are really fascinating, and something that I'm sure publishers do a fair amount of secret research into.
Reviews are far more important to new IPs though, obviously. Many people will buy a sequel based on how much they enjoyed the previous game.
I do think the most underestimated factor in game sales is word of mouth. That's generally how I believe most of my friends (who tend to be "Halo + GTA casual" gamers) buy games. I know that's how a great many copies of CoD4 were sold to them, off of recommendations by friends. Personally, I know I bought Uncharted after (in person) word of mouth, and Valkyria Chronicles after reading a great deal of good stuff about it online a few months ago (one of the best gaming decisions I've made, heh).
