The opening sequence to the film, kind of a Rube-Goldberg type slapstick thing which ends with a man almost getting drowned in his car, is a dead giveaway to the audience that this film is not going to be in the same league as the first two Supermans. It's supposed to be funny, but it just clunks along like a tank with lead caterpillar treads, and it goes nowhere fast. Apparently Lester and the Salkinds really didn't understand how slapstick stunts work, and they should have been locked in a room with a copy of "Monty Python and the Holy Grail" until they got it right.
The movie does pick up a little bit from there, and there are a few good bits that save it from being utter crap: the fight between 'good' and 'bad' Superman in the junkyard, with its denouement where Clark undoes his shirt to reveal the original hue of the costume, is quite nice. Some of the moments with Annette O'Toole as Lana Lang are decent, too - I'm not her biggest fan, but she suits the role of the small town girl that Clark left behind and fleshes it out well.
Everything else until the very end is, well, bad. Going from Terence Stamp and Gene Hackman to people like Robert Vaughn and Richard Pryor is a terrible, terrible decision and completely undercuts any sense of real portent or menace that a decent villain is meant to have in a film like this. You can't believe for a moment that Vaughn's lightweight character and his nitwit entourage would be any kind of match for the Man Of Steel, and Pryor's role, even as an innocent dupe, just isn't what this film needs - he is completely out of place here. Give me the Pryor from "BiCentennial ******" instead; THAT character belongs in a Superman film.
There is just a bit of style and flair in the final showdown scene with the Super Computer where the villains are transformed into nightmarish robot slaves and then Superman is lashed repeatedly with huge energy bolts and pulled helplessly inside the guts of the computer - you actually get a sense of conflict and drama, and the clever way Supes saves the day by using his smarts and cunning almost saves the film.
But it's not enough. 75% of the film has the same problems as the opening sequence - it just doesn't work or hold together. Unlike the first two films, the stuff that is supposed to be funny simply isn't, and most of the stuff that is supposed to be dramatic is spoiled by bad timing or misjudged performances that ruin any sense of involvement in the film.
Rent the tape or DVD once so you can fast-forward to one or two of the scenes with O'Toole, the fight in the junkyard, and the fight with the Super Computer. Otherwise, I think this movie killed the franchise - by the time Superman IV appeared, no one was interested anymore.