I just got my 580EX MkII and have had a play with it.
I don't have the Mk I to compare with, but this is a big improvement on my old 420EX (which I'll keep as a slave).
I love the battery door, it doesn't feel as though it's going to break off in your fingers like the 420, and the quick-action system on the metal foot is just great. No more wriggling/shoving/twisting/pulling to get it on/off the camera.
It's really quick to cycle, and absolutely silent. Dog-shooters will love the fact that even dogs don't appear to hear it charge (at least mine don't!)
I have yet to play with the custom functions, and I think the catch-light panel/wide diffuser might be a bit of a gimmick, particularly as I use a Stofen.