Let's state the obvious right off the bat. If it weren't for the stop-motion animation in this film, it would be simply awful. Awful acting, awful script, mediocre direction, this film has it all.
But once the monster appears on screen, none of that matters. Ray Harryhausen's animation is, as always, simply spellbinding, giving the monster, paradoxically, both a heightened reality (as it really is a physical object photographed in "real life") and a dreamlike quality. It's easy to see how Harryhausen's work set the standards for monster special effects until Star Wars and computer animation came along many years later.
This film is a particularly good example of his work for a number of reasons. There's only one monster (unlike the Sinbad/Jason/Titans movies), so all his effort is spent on that one "character". The monster also starts out small and grows huge by the end of the movie, allowing us to see it in a variety of settings. And, the fact that it's a humanoid (rather than a dinosaur or big octopus) allows it to "act" in a much more expressive manner (not unlike the original Kong).
So while this movie may qualify as little more than "MST3K" fodder as a science fiction work (did I mention how truly awful the script is?), as a piece of animation, it's a pure classic, deserving a space on your shelf next to King Kong, Snow White and Fantasia.