A lot to enjoy in this one - creepy vibe, nice use of ghost story tropes and tricks, really unnerving Last Cyberman, handled the moral dilemma well - no right, no wrong, just a discussion, a choice, and a determination to deal with the consequences - and I certainly appreciated the detail of Graham's 'ghosts'.
If I were to nitpick, I'd agree with a poster above that the guest historical figures felt a little flat, and there was a moment, and I admit I could have blinked and missed something, where it jumped to the Doctor confronting the cyberman with no explanation of how they came together. I'd disagree that Dr Who avoids changing history, though; in this ep alone two people died who hadn't before, Mary Shelley found an entirely new inspiration for her iconic creation, and Clairmont ended her relationship with Byron early, thus likely ensuring their child is never born. Probably more accurate to say they rarely to never change anything major.
Regarding the overall series, so far, I do think the main arc has been handled in a slightly clunky fashion, not terribly well-paced. Big, dramatic two-part opener, then standalones that barely touch the arc, then a bigger, more dramatic arc ep, then two more eps that barely mention it, then finally a standalone turns into an arc ep, except dealing with a relatively lesser element, and ignoring the most dramatic reveal. Can't help being a little concerned about how all this will turn up, but hopefully it's unwarranted.