Upon first seeing the trailer, I thought to myself, 'Well, that's going to be extremely funny, unless they seriously drop the ball.' Great concept, strong performers, etc.
I'm happy to report that the ball was not dropped. Funniest movie I've seen since Anchorman. I know everyone is all over Downey's performance, and it's damn funny, but I have to say that I liked Stiller even more - if only because he allowed himself to downplay (in contrast, that is) and let his fellow performers go off.