Roger Ebert's verdict is in, and he be lovin' Synecdoche, New York.

I think you have to see Charlie Kaufman's "Synecdoche, New York" twice. I watched it the first time and knew it was a great film and that I had not mastered it. The second time because I needed to. The third time because I will want to. It will open to confused audiences and live indefinitely. A lot of people these days don't even go to a movie once. There are alternatives. It doesn't have to be the movies, but we must somehow dream. If we don't "go to the movies" in any form, our minds wither and sicken.

This is a film with the richness of great fiction. Like Suttree, the Cormac McCarthy novel I'm always mentioning, it's not that you have to return to understand it. It's that you have to return to realize how fine it really is. The surface may daunt you. The depths enfold you. The whole reveals itself, and then you may return to it like a talisman. (Source)

Meanwhile, the New Yorker's Anthony Lane is less than jazzed:

Such is the way of this unusual film. It sets up, in the person of its central character, a monolithic desolation, and then spends two hours capering around it, so that we emerge, at the end, not quite sure whether to feel toyed with or ground down. No one could accuse the director, Charlie Kaufman, of lacking ambition. Few directors these days, especially in America, are willing to roll up their sleeves and grapple with existence, mortality, creativity, love, and the gulf where love was meant to be.

...There has long been a strain of sorry lassitude in Kaufman's work, and here it sickens into the morbid.

... there are three commonplaces on which it repeatedly riffs. One is what you might call the romantic-pathetic theory of imagination: any alternative reality that we design and furnish, when we conceive a work of art, is always to some extent a stand-in for the puny or pitiful one that we have been personally landed with. The second and most imperishable truth is: we grow old, and perish. And the third says: all you need is love. These are noble principles to pursue; unless the pursuit is waged with gusto, however, it threatens to slump into the sententious, and that is what happens here. (Source)

Thanks to Laurel and Stefano.