I thought it was very good but not great - probably a 7 on the IMDB rating scale, maybe even an 8 because I thought the last hour was much stronger than the first.
The first half hour or so is one solid video game - all sound and fury, signifying very little. It gets a little more interesting in the quieter moments, until an awkward Lucas line hits, or Hayden Christiansen looks more uncomfortable than dangerous. The effects are great, but because there so many special effects it seems like overkill after a while.
The final battle is pretty good, and there is a frighteningly ironic detail that shows you how Vader survives his encounter with hell.
The acting is all over the place, but it's mostly pretty good, led by very strong performances by Ian McDarmind (Palpatine) and Ewan McGregor (Obi-Wan). Christiansen is certainly stronger in this movie than in Attack of the Clones, but still doesn't quite pull it off. Natalie Portman is good as Padme but is given depressingly little to do. It would have been nice to see more of Jimmy Smits. Yoda has become the best CGI character since Gollum.
This week, we watched episodes I & II before going to the theater to see episode III, and we watched episodes IV & V after seeing Sith. The earlier movies have a kind of visual elegance to them that's almost totally lacking in the current set (with a couple of exceptions in III, and cribbing from Dinotopia in II). Some special effects gurus feel compelled to fill up the screen with just more effects. This doesn't necessarily make a better movie.
There is a wonderful scene in Return of the Jedi between Luke and Leia, where they talk about their parents. Leia has a line where she simply says "I remember my mother. She was so beautiful...and so sad," as if she'd known her biological mother, and her mother had died when Leia was a little girl (and not just when Alderan is blown up in Star Wars). I'd always hoped that Padme survived childbirth, perhaps being presented as a queen on Alderan, to raise Leia as her daughter. Since Leia is, also, raised by an adoptive mother, that line won't feel the same anymore.
I thought it was a terrible cheat to not show Annikan slaughtering the Jedis-in-training. We saw him slaughter everyone else, and the movie was rated PG-13 for a reason! I'm not advocating blood and guts, just honest, on-camera proof of what a monster Vader had become.