For me, it depends on what type of genre the game is that is being reviewed. If it's a racing/puzzle/RTS game and the general reviews have been good, like 65% or higher- then I'll easily plunk down $70. Most of the time I'm more than satisified.
Other genres I look at all the reviews and if I'm interested and it got good reviews I wait untill it hits $30 before I snag it.
The other extreme is certain types, like Sports/music/movie based games, I won't touch with a 50 foot pole (yes even guitar hero), even if they get game of the year awards and sell 50 million copies.
However, I find that 95% of all videogames are over rated anyway. I mean Viva Pinata is great, but in no way it deserves higer than 85%.