I thought it was pretty good overall. Good special effects, good fight scenes, and Anakin's assault on the Jedi temple was disturbing and moving in all the right ways. The romance was still terribly done, although the acting was a little more convincing this time. I'm pretty irritated by what was done to Padme, though. T said it best -- in the first two movies, Padme was an important character who did things. In this movie, she appeared to be little more than a housewife, her only role to be pregnant and to drive Anakin into action.
My other big complaint is that the character development was rushed. Anakin's choice to turn to the dark side was believable, but it happened way too fast -- one minute he was conflicted, and literally the next he was the Emperor's man. I think the entire trilogy would have been greatly improved if the first two movies had been combined into one and this one spread out over two, perhaps one movie about Anakin turning to evil and then a second about his betrayal of the Jedi?
Still, this was far and away the best of the prequels.
Via Jed, here is a really fascinating retcon on the events of the original Star Wars movie (I refuse to call it "A New Hope") in light of what we've learned in the prequels, particularly "Revenge of the Sith". Contains lots of spoilers for all six movies.