The second film is enjoyable, (it kicked off the SciFi channel era of MST3K) the third is more melodrama than monster movie. (Taking the Gillman and surgically altering him into a land dweller was a bad idea.)
I didn't enjoy it when I saw it as a kid, but when revisiting it in the DVD set it had it's moments, especially since lead actors from "This Island Earth" are in it.
There was a live musical version of the first movie running at Universal in Hollywood a couple years ago.