To me, the appeal of the movie is obvious. It's hilarious, horrific, bizarre, melodramatic, self-serious, grotesque, childish and completely dependent on a movie I enjoy almost as much to make any sense. And it features a completely unnecessary opening framing device, clearly there to please the creative staff and no one else.
If the original Frankenstein film fails to capture the book, this one more or less throws the book away while also laying claim to it, using small portions and ideas of the book to tell a new and unnecessary story, but somehow a deeply fulfilling one - and in the process makes a double-bill of Frankenstein and Bride of Frankenstein, really, the best way to see both pictures and consider it one long project with an interruption mid-way through. Upon returning from that break, you'll notice a change in tone of the film to a grander sense of scale, weirder characters and poor Colin Clive seemingly more wrecked than in even the first movie, all the while everyone else seems to be having a grand old time putting on a show.