My background:
I really liked the original trilogy, though the Ewoks in Return of the Jedi were annoying. I thought Episode I sucked outright, and Episode II was better, but was still pretty mediocre.
Episode III was alot better. Not up to the old movies, but alot better and at least in the same ballpark in terms of all-around quality as the old movies.
For one thing, Episode III has more events happen to more people that are more significant to the overall story than the previous two movies combined. So it starts with the potential to be alot more interesting. I'll go as far as to say that Episode III shows Episodes I & II to be almost totally unnecessary to telling the Annakin/Vader story.
Annakin and Obi-Wan actually seem like they were buddies at the beginning. The big opening action scene with them reminds you of the fun factor of the old Star Wars action scenes. There are some good one-liners here, too, and the little bits of "situational" humor work better.
Palpatine was just alot of fun to watch, he was such a subtle evil weasel when pretending to be a good guy, and when he comes out in the open, he is a COMPLETELY over-the-top maniac. You could make a GOOD argument that this was way overdone, but it was still fun to watch.
Annakin is improved quite a bit, Padme is still basically furniture with boobs. Obi-Wan is ALOT better, I'd say Ewan McGregor pretty much nails the character this time. Mace Windu coasts mostly on resudial Samuel L. Jackson bad-ass fumes, but is still OK. The CGI is alot better in this fim, but is overused and sometimes headache-inducing even on DLP.
No Jar-Jar at all, no bad child actors, less bad romance, less bitchiness and more evil from the future Vader, no C3P0 bad pun/slapstick "humor."
My overall ranking:
Star Wars: A
Empire Strikes Back: A
Return of the Jedi: B+
The Phantom Menace: C-
Attack of the Clones: C+
Revenge of the Sith: B
