One of the best of the movies with Ray Harryhousen's Dynamation special effects. The Greek myth(s) about Jason were pretty well represented, in other words a great story. I only downgraded it from 5 stars to 4 because the story 'just ended' with Jason and the heroine sailing off into the sunset. Does Jason even get back home to Thessaly? How does he even get back through the Clashing Rocks on his way back? Does he finally confront the usurper-king, and win back his kingdom? In the original myths he does, but the movie's end leaves you guessing. Including an actual (or Hollywood) ending to the story might have added as much as 30 minutes to the movie, but since Jason encountered no more monsters on the way back, there was not much for Harryhousen to do.
As an aside, for those of you who might be wondering just where the 'Clashing Rocks' were located, I looked it up for you. The actual location is the Bosphorus - that very narrow strait between the Sea of Marmara (Constantinople) and the Black Sea. Colchis was said to be somewhere on the Southern shore of the Black Sea. Just to avoid any confusion, the Hellespont (now the Dardanelles) is the narrow strait between the Aegean Sea (Greek side) to the Sea of Marmara. Troy was supposed to be located at the entrance to the Hellespont.
It is also said in myth that floating rocks known as the Symplegades or Clashing Rocks once crushed any ship that attempted passage of the Bosphorus until the hero Jason obtained passage, whereupon the rocks became fixed, and Greek access to the Black Sea was opened. Now you know how Jason avoided being crushed on his way back to Thessaly.
History and myth aside, this movie is one of the best of Harryhousen's efforts, told a great story in only 101 minutes, and is therefore highly recommended. The more recent re-makes of the Jason story, as well as the Perseus vs. the Gorgon myth (Clash of the Titans) have better effects (at least more modern), but their stories just don't hold together as well.