Well Bill Hunt has his review up for Episode III. Interesting reading. Of all the reviews I have read so far, I know I can trust his judgement to at least have some objectivity. He does say Episode III is easily the best prequel (no surprise there) and that Hayden Christensen does a much better job in this film (one of my big worries).
www.thedigitalbits.com/

For those who don't want to read the whole review, here is his revised grades for all six films:
The Phantom Menace: C
Attack of the Clones: C+
Revenge of the Sith: B+
A New Hope: A
The Empire Strikes Back: A+
Return of the Jedi: B-