IGN's review: http://pc.ign.com/articles/930/930763p1.html (Apparently Firefox is giving some people issues on IGN, try IE or another browser if so)
Video review: http://pc.ign.com/dor/objects/818215/turtle-rock-project/videos/left4dead_111708.html
IGN - 9.0!
Also, Official Xbox Magazine has reviewed Left 4 Dead, giving it a 95/100.
edit: Gamespy gives it a 4.5/5! http://pc.gamespy.com/pc/turtle-rock-project/930737p1.html
LEFT4DEAD411.COM
Bet with disolitude: Left4Dead will have a higher Metacritic rating than Project Origin, 3 months after the second game's release. (hasn't been 3 months but it looks like I won :-p )