I actually have quite a bit of respect for Roger Corman, especially because he's given a lot of people their start in movies: Jack Nicholson, Jonathan Demme, Ron Howard and John Sayles among many, many others. But this one is an embarrassment to nearly all concerned: awful dialogue, no atmosphere, anachronisms galore. I think possibly the reason Corman hasn't directed anything since this is that he realized he couldn't do it anymore. There are a couple of redeeming factors: the performance of Nick Brimble as the monster, who in this interpretation is agile, articulate, and somewhat complex; and the Frankenstein story itself, which is closer to Mary Shelley's original than the vast majority of other treatments. (In my opinion the original story has never really been done justice in film; the closest is "Bride of Frankenstein.")
Review of Frankenstein Unbound
