Superman III - good and bad
Good:
Evil Superman vs Clark Kent was obviously the highlight of the movie. As was Superman turning bad after being exposed to Kryptonite (flicking peanuts at, then melting a mirror in self-hate when seeing himself). This is the sort of darker, serious stuff Superman III needed more of, and was surely the very sort of thing that helped got Donner fired because Superman II's use of villains was far, far darker and more menacing and a lot of it was excised or re-shot to be softer (which in turn made Superman II less of an event when much promise was made over how terrifying General Zod should have been!!)
Superman III clearly vindicates how right Donner was all along, in terms of how the franchise should be made.
Bad:
I missed Lois Lane. What the heck happened?
Why is the phony kryptonite green when it was established for decades that red kryptonite made Superman act mean and nasty? Gus could have accidentally figured out red kryptonite; making it green should have had Superman looking at it in terror because he's seen kryptonite before and how it would hurt or kill him. The fake stuff just as Superman looking dumbfounded over a rock that is more ugly than precious in appearance, and why would he? He doesn't care for the bling made by humans...
Really, turning Superman into a comedy franchise?! This is the final proof that Superman II should have been left to be finished with its original vision first. III is what killed the series, since IV would also be impacted by the disaster III put out.
The supercomputer, made of Earth parts and software circa 1983, couldn't begin to do all that... never mind turning it into a half-comedy piece...
It really jumps its shark the moment street signs become animated to fight one another... who was insulting whom when that travesty within a travesty was written out to be produced?
Richard Pryor's talents were wasted.
So were the talents of Christopher Reeve and everyone else in the film.
In summary: Because of III, the franchise would ultimately lose far more money... than firing Donner, nixing Brando's scenes, etc, etc, in an attempt to save money...