The first thing to say is that it's interesting. The second thing to say is that it's more interesting than good.
Cushing's Frankenstein is now the resident doctor in a mental asylum, albeit theoretically an inmate. What's more, he's a nice guy! Still up to his old tricks, but he now has a practice and seems to be doing a good job, if one overlooks the small ethical matter of occasionally using his patients for spare parts. Nevertheless, before seeing him chop anyone up, we get an astonishingly lengthy sequence of him doing the rounds of his patients. This certainly isn't a Frankenstein who'd rape his landlady or murder anyone in cold blood, as we've seen him do in the past. He's even become philosophical, musing over the past and how long he's been doing this... which is certainly true if only in the sense that Cushing's Frankenstein first appeared in 1957 and this film came out in 1973.
However there's a more fundamental script reason why Baron Frankenstein seems less abrasive this time around. The writers screwed up, basically. Frankenstein's sidekick  in this film is basically another Frankenstein. He's a fan, a young doctor who'd read all his published works, was trying to replicate his experiments and got sent to exactly the same institution he did. That I have no problem with, but unfortunately by now Peter Cushing had left such a stamp on the role (as seen in the Ralph Bates version in 1970) that it never occurs to the scriptwriters to give this wannabe a contrasting personality. This could have been fabulous! Look at all the different actors to have played the role! Stage, screen, radio... we could have had some fascinating clashes of approach and personality. But no, Cushing's foil in this film is an icy clone of himself who instantly accepts his subordinate status and becomes a rather chilly doormat. There's no conflict, no dramatic contrast. The Baron has no enemies, no one to fight. Thus he can't shine.
As I said, it's an interesting approach and one I can appreciate. The Baron evolves enormously over the course of these films and I like the way he's gone through his various stages of Scary, Dedicated and/or outright Evil to finally end up as this tired old man doing his best in a lunatic asylum. However it's undeniably less memorable.
Then there's the shocking difference in Cushing himself. In 1971 his wife died and Cushing, by all accounts the kindest and most sensitive of men, was pretty much a shell of a man for the rest of his life. The following year he was quoted in the Radio Times as saying, "Since Helen passed on I can't find anything; the heart, quite simply, has gone out of everything. Time is interminable, the loneliness is almost unbearable and the only thing that keeps me going is the knowledge that my dear Helen and I will be united again some day. To join Helen is my only ambition. You have my permission to publish that... really, you know dear boy, it's all just killing time. Please say that." You can see it in his performance here. It's practically screaming at you. The spark's gone out of him.
In this context and playing this particular characterisation, though, that works.
 - Cushing's Frankenstein generally needed a sidekick, thanks to the inspired plot device of crippled hands.
There are incidental pleasures. Patrick Troughton appears (briefly), as does the lovely Madeline Smith although disappointingly she keeps her clothes on. More distracting than them for a Doctor Who fan is the asylum's director, whom I recognised without being able to place until I saw the closing credits. It's John Stratton, playing it just as fruity as he did Shockeye in The Two Doctors. That's "fruity" in the British sense, not the American. There's also some icky brain surgery which had me cringing on the sofa despite it obviously being a dummy they're cutting up, but then flooring me with a throwaway visual gag. It's amazing the fun you can have with a brain!
The monster looks silly, though.
Overall, fascinating as the culmination of the evolution of Hammer's Frankenstein. I really enjoyed it. However without that context and a knowledge of the history of Cushing's Baron, as a film in its own right I don't know if I'd call it a success.