"Voyage to a Prehistoric Planet" should be excruciating, but miraculously it's a pretty fun flick - provided you're into the B movie thing (and if you aren't, why on earth are you looking this "masterpiece" up?)
First, a little historical note. Although the American version of the film features the great Basil Rathbourne and that monotonous beauty Faith Domergue, these two thesps were in fact added in to the original footage in order to increase its appeal for a U.S. audience (the movie is actually Russian - or maybe Swedish). They aren't supposed to be there, and you can sorta tell, since they never get involved in the action. Sadly, they end up dragging the movie down, since all they do is communicate with each other by radio, slowing the action to a crawl with lots of pointless dialogue like, "I hope everybody's okay down there on Venus. Keep your fingers crossed..."
Now for the rest. Just about every scene in the movie falls into one of three categories:
(1) Tedious (2) Silly Fun (3) Genuinely Interesting
For #1, you've got lots of milling around in quarries and spaceship sets. For #2, you've got cool rubber monsters and the world's lamest aircar, which waddles along slower than your granny could hobble. For #3, you've got some cool cryptic references to the Venusian civilization, which pretty much remains a mystery for the entire film. I was particularly impressed by the single, indistinct, mysterious shot of the native aliens, and by the carving hidden in a hunk of rock. Too bad the whole movie doesn't deal with tracking down clues about the alien civilization, but alas, it's mostly concerned with techno-talk and survivalism.
Overall - quite good, if you're in the right company.