Image via Yahoo! Movies
Update: Well. I have to say, that was one of the most interesting theatrical experiences I've had in quite some time. The dialogue was, for the most part, inexcusably bad. (One of Yoda's lines in particular was... gyecch!!) However, throughout I found myself trying to look beneath the painfully stilted banter at the story that Mr. Lucas is trying to tell. In wordless emotions and images, it's quite powerful. I would very much like to see what a more capable writer (i.e., one who hasn't been busy running a multi-billion dollar company for twenty years) could have done with the script. Lots of potential there - if it weren't for the dialogue, I would say that E-III could have been the best of all of the movies.
No spoilers here, I don't have the energy to go into that level of detail. Suffice to say, though, that this semi-final installment greatly clarifies the purpose of the previous two movies. The psychology of Sith would be greatly diluted without the backstory of Phantom Menace and Clones.
In summary: Strong story, excellent use of special effects, and definitely better than E-I and E-II. Flimsy execution/acting/dialogue prevents it from being truly great, but overall it's a worthy contribution to the saga.