I'd cite "Batman Forever: The Red Book Edition" as a film that takes a seriously flawed movie, and improved it greatly... although I wish the editor hadn't gone so crazy with the desaturation button.
I think "Doom: The Possessed" cut was great... well, great in the sense that it took an unwatchably bad movie and made it a fun bad movie.
One of the more creative ones I saw was 'Superman: Redeemed', which took the action from Superman 4, and mixed it with the basic plot of Superman 3. While better than either original film, it succeeded in making me realize even the good parts of those films still sucked.
Some just seem pointless. 'X-Men: Requium War' combined all three X-men films into one three hour epic. It worked plot wise, but seemed pointless somehow. The X2 parts still ruled, the X3 parts still sucked, and I still don't know what the title means.
On the otherhand "Marvel: 24" was just a hoot and a half. Balls to the wall fun, no explanation needed.
When it comes to Star Wars, I have a tough time understanding some of the edits I've heard of. One thing might be I simply don't get major love for minor villains. Boba Fett did his role, looked cool, and died a goon's death. Darth Maul and Greivous both served their purposes. I don't need to see a Robot-Tarkin at Cloud City to make me appreciate the supporting villain who died previously.
I think what Phantom Editor did was great... he edited the movie as an editor. He didn't try to re-write it... and the films benefited greatly from that.
One thing that gets me is how many changes are justified as 'truer to the spirit' or something rather than 'my opinion as a faneditor.' And usually, in Star Wars and most films, 'truer to the spirit' means darker and more violent, with no kissy parts.
Temple of Doom? Cut the funny bits to be 'truer to the spirit.'
ROTJ? Less funny bits to be 'truer to the spirit'
The Spirit? Less funny, more punchy, to be truer to the Spirit.
Spirit: Stallion of the Cimmaron... you get the idea.
Couldn't it easily be said the ESB is not true to the 'spirit' of Star Wars, since it's darker and depressing? Yet no one edits it to be lighter in tone (an upbeat ending somehow?)