Saw it. Liked it. Not a fucking clue what it's actually supposed to mean just yet, but it's incredibly arresting and watchable. Manages an interesting trick with a minor supporting character played by Jennifer Jason Leigh whereby she evolves into about the closest thing the film has to an outright villian - and a fairly reprehensible creature of one at that - entirely offscreen. The character turns up about four times very briefly, but somehow second-hand information from other characters fleshes her out more than some of the more frequently-seen players. Neat trick, if nothing else.
The basic idea here is a middle-age-to-death story of a struggling theatre director (Philip Seymour Hoffman) who opts to use a McArthur Genius Grant to stage a "massive theater piece." His scheme involves renting a MASSIVE warehouse, building a to-scale recreation of New York City and populating it with actors playing himself, his aquaintances and just random people in an autobiographical recreation of... well, everything. Soon enough, there are actors playing actors PLAYING ACTORS, the story turns so inward that a set of the warehouse goes up IN the warehouse and the whole thing seems to consume everyone involved. It may help to note that the world outside the warehouse is actually MORE surreal: One character buys and lives in a house that is perpetually on-fire but never burns down, a therapist seems to operate through some form of precognition, and the Director finds the actor to play him in a mysterious fellow who has been following him around secretly his entire life. Oh, and there are small implications throughout that even the "real" parts are being stage-managed in some way, like Hoffman pausing to squirt in artificial-tears before a big breakdown scene.
So, yeah... it's a Charlie Kaufman movie. I'm not sure it all fits together as well as some of his previous scripts, but it's incredibly interesting and full of BIG ideas to chew on. Oh, and a topless Emily Watson. You CAN'T go wrong with that. (She's soooo much hotter than she gets credit for most of the time.)
Incidentally, for what it's worth, a "Synecdoche" (Sin-Ech-Doh-Key) is when you use a part of something to refer to a whole, usually as in a group (i.e. referring to an army as "500 guns" instead of "500 men WITH guns.") I've not the foggiest what it means in the context of the film-proper, however, aside from a rhyming pun on the central location of Scenectady, New York. Make of that what you will.