The actors may not be great, the plot may meander, but the film's stop-motion/live-action integration achieves remarkable effects. It's not that the creatures look realistic. It's that they look just realistic enough while maintaining a look that also suggests their otherworldly nature within the narrative.
Jason and the Argonauts contains two of Harryhausen's greatest achievements -- the giant bronze 'robot' Talos and the great skeleton battle. We also get a nifty battle between Jason and the Hydra and a somewhat disappointing sequence involving Harpies, who never seem to be integrated as effectively as the other stop-motion creatures. Oh, well.
Jason and his crew of merry Greeks search for the Golden Fleece, Hercules screws up, and many great battles are had with monsters while the Greek gods help or hinder Jason in his quest. The battle between Jason and several reanimated skeletons occurs at the end of the film, and it really is a show-stopper. Highly recommended.