Two reviews today; first one's from my current fave blog, io9:

By the time Kaufman calls in a modern deus ex machina, I was ready for this long strange trip through psychological and English and drama theory to end. But Synecdoche will have as many ardent fans as it will befuddled viewers calling bullshit. It's easy to see that Kaufman is trying to make deep investigations into the human psyche: the themes of life, death, war, family and romantic love are writ large and sometimes literally preached at us or given a special monologue. But the movie is impaired by how much of a free reign its writer-director has been given — his hands are in too many pots. Its run time could be nearly halved and still maintain the parts that are the most affective and revelatory.

See Synecdoche, New York if you love Charlie Kaufman's uncommon worlds, if you have a fond taste for the bizarre, and the willingness to give up all narrative bearings. See it especially if you enjoy endlessly ruminating on the nature of existence. Just don't see it with your friends who have short attention spans, or anyone not keen on all that's meta and much too self-aware. (Source)

Meanwhile, Jim Rohner says pretty much the same thing:

The main appeal to such a brilliant and unique writer is his ability to open our eyes to new perspectives on our world, but with Synecdoche, New York he seems so lost in his own world that anything said or portrayed within his two hour fantasy only applies to the cinematic world he created; spiraling wildly out of control with each minute that passes.  While it's absurd to assume Kaufman is “too into himself” (he's much to humble/introverted of a man for that), it's not absurd to think that he, clouded with so many tangents of the fantastical, cannot see the trees because he's focused on the forest.  The end result is a two-hour aimless trudge through the existential that feels like three hours and fails to live up to the expectations of Kaufman's directorial debut. (Source)