No really, not so bad. Lucas manages to resolve a few of the gaping holes in the plot of the original series and I found the dialog less painful than expected. It's probably the second best movie of the six, behind The Empire Strikes Back.
No spoilers, but a few side notes. I think I saw the Millenium Falcon in one scene, it's entering a massive docking area towards the lower level. The relationship between Chewbacca and Han Solo is neatly inverted; perhaps Chewie was not on Han's ship by accident. R2D2 knows more than one might suspect. Lastly Lucas compensates a bit for Episode IV's (first movie) dismissive portrayal of adoption; a change possibly related to his own adopted children.
Prior to the movie, I recommend reading this ver well written and amusing blog - The Darth Side: Memoirs of a Monster: 20050403. I'm now convinced the author is not really in the pay of George Lucas, but he seemed to have some advanced knowledge of some of the minor secrets of 'Revenge of the Sith'.