For many people, this is THE Superman movie. There is no other. Every sequel falls short, every subsequent attempt at reaching this level of greatness looks pathetic by comparison.
Now, for me, I don't know that I'd go that far in praising this movie. I enjoyed it, for sure. The effects are very good (at least, by 1978 standards, and even in some cases by today's) and the acting was extremely good-- I see why many people consider Christopher Reeve the best Superman ever, because he went so smoothly between the superhero and the awkward journalist. I also enjoyed Gene Hackman's Lex Luthor-- polite, intelligent, funny, egomaniacal, but manically twisted-- and Margot Kidder's Lois Lane, who lacked the intense toughness that is sometimes associated with the character, but still frequently pulled out a hint of a hard edge mixed in with her crush on the superhero. Finally, who could not love Marlon Brando's portrayal of Jor-El, Superman's father-- the scientist, stymied by politicians, who knew his end was coming, who saved his son and sent him all the knowledge and guidance he could need.
In my opinion, the acting, particularly of those four, made the movie. I found the plot sometimes jumpy and hard to keep up with. At points it was almost like the movie assumed that we knew things that I at least didn't know. Some events seemed totally random, at least at first and even second glance, such as Clark's adoptive father's death and Clark's decision to leave his small town. I felt like certain scenes could have been developed more, essentially. Then again, I doubt this was ever meant to be a fully stand-alone movie-- they surely had sequels in mind to answer more of the questions that were left lingering.
All things told, barring the jumpiness, I really enjoyed the 1978 Superman. Reeve, Hackman, and Brando helped alleviate the sense of fragmentation in the plot because their acting delivered the story with clarity and fun. Oh, and who could forget John Williams's classic score to this film? His theme for this movie, as for many others, has passed into the status of musical legend. Bottom line- this is still a classic (and defining) representation of one of the ultimate superheroes.