I don't see many midnight movies, but when some friends at work decided to go see Revenge of the Sith as soon as possible, I tagged along. I missed seeing Brazil at a midnight show recently, but could swing this because I would get about 90 more minutes of sleep. ;)
There isn't really a plot to spoil. Really, everybody who is going to see it already knows what has to happen, since the next 3 movies came out first. You know Anakin falls to the darkside of the Force and becomes Darth Vader; he gets horribly injured and needs his signature all-black suit; Palpatine is the Sith Lord, and he seizes control of the Empire; Padme gives birth to a boy (Luke) and girl (Leia); all Jedi except Yoda and Obi-Wan Kenobi are killed. That covers most of it, so what is left is the story surrounding all these events. There is quite a bit of fighting: Obi-Wan versus Anakin, Yoda versus Darth Sidious, Obi-Wan versus a cool looking four-armed robot - General Grevious, and so forth.
Anakin's fall to evil was even setup well: the Jedi Council wants him to spy on the Chancellor, while the Chancellor wants him to be his representative on the Council. He has conflicting loyalties, and Palpatine reveals who he really is in a bid to gain him as an apprentice. Anakin reports Palpatine is the Sith Lord, and when the Jedi go to arrest him, Mace Windu gets a little carried away and tries to kill him. Palpatine defends himself and kills Windu, and then uses that to show Anakin the Jedi aren't to be trusted. Instead, if Anakin would join Palpatine, he could help end the war and also learn dark side powers that will help save Padme from death. So, Anakin switches sides.
Now you'd think Anakin could step back for a minute and say, hey, you're the guy responsible for the war in the first place, are eager to grab more and more power, want me to go kill off a bunch of people to clean up... this doesn't seem right. So while his fall from grace was a bit rushed, it was at least reasonable. Ironically, and predictably, his shift to the dark side causes Padme's death - first an injury by a force choke hold, and then she loses the will to live after a difficult childbirth.
Other nits include the romance between Anakin and Padme, which seemed a bit forced... Hayden Christensen and Natalie Portman don't seem to have any believable chemistry. But overall, the movie was pretty good and I did like it, much better than The Phantom Menace and Attack of the Clones.