All this and Superman two
It is understandably a little rough in places. Restored footage was scavenged from all over, including screen tests, so you need to be a little forgiving. Some scenes are very underdeveloped, and they could have done with keeping in a little more of the footage from the release version in places, though I am guessing they were left out because they would clash too much - I'm thinking particularly of the scenes in the Niagara Falls hotel room. And I think they were right to redo that scene. I'm not going to give away any spoilers but how stupid is Superman to fall for that?
I can't do a big comparison between this and the released version. I'm sure there are plenty of those already available, and I haven't seen the old version in ages. I caught about half of it on TV when Superman Returns came out, but that's it. But I noticed one thing which has always irked me has been removed. During the big fight scene in New York the drama and tension of the scene is constantly undermined by the inclusion of bits of physical humour by the victims as they are getting blown away. I am delighted that these are gone. I think the whole thing would have been a far better film if this version had been properly finished. Except possibly for the ending.
I have a problem with the ending. If this was what Richard Donner intended, then he's an idiot, and I can see why the production was taken out of his hands. It's like an athlete running a good race and then three yards short of the finish line he trips over his own feet and lands face first in the poo.
I don't think it counts as a spoiler, since it happens after the villains have been defeated (what, you thought that in the original version they would win?). In footage taken entirely from Superman I or from earlier in the movie but shown in reverse, Superman causes time to run backwards so that everything that was damaged is fixed and Zod and co. never escaped the Phantom Zone.
How many kinds of stupid is this? If he could do this all along, why did he bother to fight them at all, or trust to a risky strategy to deceive them? When he saw how much trouble they were why didn't he go give the planet a quick reverse spin? Why does he need to fight any battle at all if he can solve every problem with a quick time reverse? Of course in the first movie I always read it that he was only sending himself back in time (flying faster than light and all that), and that's a very different proposition to reversing time for the whole world. Why would any director give the same ending to two movies?
There's reason to think this wasn't what Donner intended (stuff Superman does immediately beforehand in restored footage becomes totally pointless if he is about to go ahead and undo it, and how can the guys in the diner recognise Clark afterwards?), and I can only guess that this part of the movie was never made, or is completely lost, and this was done to cover the gap. It fails.
This may not be quite the movie Superman II should have been, but it's as close as we're going to get.