Saw this over the weekend for the first time. I'm a big fan of Charlie Kaufman (Adaptation and Eternal Sunshine are brilliant), and although I enjoyed this and respect it, it's probably the least of his films. The novelty of a living play in a warehouse would have been enough to set this film apart, but add onto that the burning house, the Apocalyptic New York, Sammy following Caden for years without explanation (and Caden accepting that), and the bizarre psychiatrist...
The 8 1/2 comparisons are apt, and the entire film may very well be Caden's fractured psyche (as Randlett states above) in retrospect piecing his life back together. Still, there needed to be a grounding element to bottle all that weirdness: for instance, the office space in Being John Malkovich is where the film diverges into surreal, and Joel's fading memories in Eternal Sunshine are contrasted with what's happening in the real world (Dunst and Ruffalo hijinx). The warehouse could have grounded this story, but the weirdness dominates every aspect of Caden's life!
Overwhelming is the word here. I'd definitely watch it again, but the problem is there's at least three high concepts here deserving of their own film that get crammed in together.
Little nitpick as well: why is Caden deserving of the MacArthur genius grant? All we see is him directing an off-Broadway Death of a Salesman with a slight twist. His first wife is a world famous painter, but more explanation of Caden's fame and/or accomplishments was necessary. Unless it was all a delusion, Caden secretly believing himself a genius worthy of acclaim, and that certainly is when the film shifts into overdrive...