On the down side,
On the up side, I just don't care -- IMO, as a movie, it kicks serious butt, and that's really all I ask of an adaptation. The pacing is a delight, starting a tad quietly but getting relentlessly tenser up through the climax. The script nicely intercuts all three stories, which is certainly different from the way the book runs, but does a good job of juxtaposition. It has just enough humor to keep things from getting unpleasantly grim. (And yes, the special effects are every bit as remarkable as they are billed -- Gollum is the first synthespian I've ever seen that just plain works.)
And the movie demonstrates, beyond a shadow of a doubt, that Gandalf is a thousand times cooler than Yoda.
That latter is actually a serious point, in a sense: the film throws into sharp relief just how bad most fantasy & SF films these days are. I mean, yes -- we knew that Star Wars had gotten bad. But it's just plain pitiful compared to this. I'd forgotten how good epic storytelling feels when it's done right: literally inspiring, getting under your skin.
Definitely the most fun I've had at a movie in a long time. See it.