Good for the time, but it has not aged well
When I saw this as a kid, it was incredible. I loved it. It is still entertaining, and Christopher Reeve gave an awesome performance (as well as the others) and the magnificent score by John Williams is so powerful. However, the movie itself has not aged well.
First, the special effects. While some were done well, others have aged terribly.
Second, the script. It had too many glaring plot holes: turning the world back, and Superman "fixing" the San Andreas Fault Line.
Third, Lex Luthor. No offense to Gene Hackman, but he played it too comical and not sinister. That is what Luthor is, was and will always be: a cold, calculating bastard, not someone who monologues while throwing in jokes every now and then. This is what truly takes away the conflict between the two. It is also the greatest weakness of this film. It's enjoyable, still, but in retrospect, it fails to capture the timelessness a spectacular movie.
Of course that's my opinion, I could be wrong.