(Spoiler notice: I'm going to try to avoid any major spoilers, but no promises. If you've seen Episode IV: A New Hope, you pretty much already know the ending.)
George Lucas had a lot to accomplish in one movie. It looks like Anakin and Padmé just got married at the end of Episode II. Anakin's a bit angry, but he still hasn't joined the Sith. Thus, in this movie, George Lucas must:
- Kill off Count Dooku. (What a terrible name.)
- Kill off Mace Windu in a manner befitting Samuel L. Jackson.
- Have Luke and Leia conceived
- Reveal Supreme Chancellor Palpatine as Darth Sidious
- Have Anakin join the Sith.
- Turn Anakin Skywalker into Darth Vader.
- Have Luke and Leia born
- Kill off Padmé. Have Luke and Leia delivered to their respective locations.
- Kill off all the Jedi.
- Mourn the Jedi after they die.
- Fight and resolve the clone war.
- Send Yoda off to his swamp.
- Turn the Republic into the Empire.
- Build Death Star I. (If there's time.)
- Kill off the trade federation guys. (If there's time.)
- Kill off a bunch of people who I can't think of off the top of my head.
There also have to be a number of prolonged action sequences and at least one "look what I can render on my computer." Ideally, there should also be an independent plot so the movie is something more than Things that Happened Between Episode II and Episode IV: The Movie. Accomplishing all of this would probably take three or four hours. Lucas accomplishes this in 180 minutes. Well, he accomplishes most of this. He doesn't accomplish the independent plot.
Thus, there is little to say about the movie. It's much darker than the previous ones. You already know the ending. Anakin goes to the dark side. Thus, the only questions remaining are 1) why, 2) who kills Mace Windu, and 3) how does Padmé die. All are resolved satisfactorily and there are no big surprises.
If Lucas had been thinking ahead, he should have cut some of the horrible-dialog-petting scenes from Episode II and appended the first scene of Episode III to that movie. That would have broken Episode II at a more logical place and given him a bit of extra time to develop a plot for this movie. Watch this movie--but if you don't like Star Wars enough to go to a midnight showing, you can afford to wait until the weekend. Or until you feel like going to a movie.