I thought Ep. III was pretty decent, but that's probably because the first two movies lowered my expectations a good bit!
IMO the first third of the movie was pointless. Just a loose connection of totally unbelievable action sequences, featuring impressive but unconvincing CGI and lacking any kind of dramatic events. After that the movie got a good deal better and I really liked the way Obi-Wan played out. In the end I could really see some sort of connection between the Obi-Wan of Ewan McGregor, and the Obi-Wan of Alec Guinness.
Some things still stand out as particularly stupid. Like that noisy lizard Obi-Wan rides on. Yeah, that's what I'd choose to sneak up on the people you wanna spy on, NOT! Why do I have a feeling this and other things in the movies were basically just added to have another toy in the stores? Still, overall the 2nd half of the movie had a decent dramatic arc to it and worked out better than I expected from a continuity POV. Ignoring that 180° turn from arrogant prick to total mass murderer within a matter of minutes (yeah is was set up even in Ep. I, but the transition was still not believable).
I also weep at all the wasted opportunities to use the force. The original trilogy showed many subtle ways in which the force could be used, it made it fascinating and cool to be a Jedi. Yet in the prequels all it seems to be good for, is to turn Jedis into jumping, lightsabre-swinging ninja rip-offs. Lightsabre duels could have been fare more dramatic too, if used less frequently.
Throghout the whole trilogy, I couldn't get rid of the feeling that the story in the background was actually pretty good, in a nerdy SW fan kind of way, but the translation of those events into a dramtic piece of screenplay was just totally screwed up. Dialogue in general isn't Lucas' forte, but what he delievered in those 3 movies is truly horrible. So much potential, characters and actors wasted. The direction and editing was uninspiring too, Lucas is really a far better producer than he is a writer/director. Why, oh why, didn't he do the prequels the same way he did the original trilogy, hire a director and writer and just control everything from the background?
Well, at least the SW prequels improved from movie to movie and ended at their highest point. Unlike some other big-ass Sci-Fi trilogy I could name, which was the other way around ... *cough* Matrix *cough* ...