Without spoilers, I can say that it was disappointing, even though a real step up from Ep. 1 and 2. It alternated between being too slow and too fast; it skipped over some parts that could've used more depth, and it focused too long on scenes that didn't need it. The dialogue was, as to be expected, terrible. The action scenes were the best from any of the movies, but flash is only worth so much in the end. The acting was so-so; you could really feel Lucas's hand crushing any chance the actors had to actually do well by the already weak script. The fall of Anakin is... weak, at best, and forced, and way too fast. Amidala turned from a strong ruler into a lovesick little girl. And a lot of things just didn't entirely seem to... line up, I guess. And Hayden Christiansen's acting chops have not improved, although watching the series, I'm inclined to say that may be Lucas's fault as much as his own.
Having said all of that? If you, like everyone else in the world, have been following the series for this long, you have to see it. And it has a lot of enjoyable moments. There were multiple scenes where the theater was cheering, and sometimes laughing (not always with the movie, if you know what I mean). And for the first time, I forgave them for making Yoda CG - they finally got it right. Really right. The fights were, at times, spectacular. The special effects and visuals were often stunning. The music hearkened back to some of the classic themes of the original trilogy. And a few people - Ewan MacGregor and Samuel L. Jackson, in particular - managed to do more with their characters than they had any right to, judging by the rest of the script.
The movie was fun, and an essential watch if you've already made it this far. I'd give it a 3.5 out of 5.
One side note: I'd recommend watching the animated mini-series (The Clone Wars) that was recently released on DVD; it explains some of the missing history and fills in some blanks between Episodes 2 and 3, which skip quite a bit. Unfortunately, there were two mini-series, and the second is not on DVD yet - so I've still only seen part of the in-between. Oh well.
Update: Oh yeah, one thing... about the violence. While it was darker and more violent than the rest of the series, esp. the scene towards the end... the burning scene... honestly, most of it wasn't nearly as bad as I was expecting, from what I had been told. For instance, the "youngling" scene (such a stupid word) - it was all suggested but not shown. From what I had been told, I thought it was actually going to be shown on screen. And most of the violence was against robots, not people. The people violence was mostly obscured or just suggested rather than shown. So it wasn't nearly as graphically violent as I was expecting.