I played it a my friends house yesterday. I managed to collect the first three stars in the short time I spent playing the game, and in this short time I found the game to be very enjoyable. Obviously I have not played enough of the game to form a critical opinion, but if it had to summarize my feelings into a score I would give it a 9/10.
To tell you the truth, I am surprised to see the game receiving such high scores. I enjoyed what I played, but in no way was I blown away by anything. The fact that it has a higher average score than LoZ: Ocarina of Time indicates that it may be overrated to a certain to degree. Games like LoZ: OOT and other groundbreaking games (Super Mario 64, Half-Life, etc) were mind blowing when they were released, establishing conventions and mechanics that other games in the genre would build from. In that sense, the critical reception of Mario Galaxy reminds me a lot of another game that was released recently; Bioshock.
When I first played the Bioshock demo I was blown away, it looked and played awesome. After the demo came the reviews, IGN was first with a 9.7 and then a slew of other sites / magazines followed with similar scores. With this came the massive hype, sites like NeoGAF had large threads dedicated to basically hyping the game as the best thing since sliced bread. It held a 97 something score for a while before being knocked down to its current standing. After renting it I would definitely say that it was subject to some serious over hyping. Thats not to say Bioshock is not an enjoyable game, just that I do not believe it deserved all of the hype it had when it was released.
Anyway, I need to play more Mario Galaxy before I continue with the the notion that it may be slightly overrated.