Pretty good. He's a clever fellow with some intriguing ideas but he doesn't quite have the same storytelling genius that nails you to the page as Asimov, Bradbury, Clarke and Heinlein do. One thing that is strikingly good and classic SF is that in some of the stories you're not quite certain which side is the "monster". In some cases such as The Sea Thing he may have intended it to be clear cut but modern readers may swing the other way. Vault of the Beast is also notable for practically being an early version of "The Terminator 2", so much so I wouldn't be surprised if it didn't actually inspire that.