Wonderful special, even if it wasn't quite I expected or even hoped for. Still, it was tremendous fun and it was especially delightful to see Mel back in the action of everything. With her working with UNIT now, I hope we get to see more of her and perhaps even a Mel and Ace reunion. I loved hearing about how Sabalom Glitz lived out his days, even if it was surprising that Mel stuck with for so long.
The first half of the episode was wonderfully creepy with all of The Toymaker's dolls and games, but I do wish the episode had leaned more into those scenes instead of essentially abandoning them after The Doctor and Donna fled the Emporium. The Toymaker's song and dance intro was perfectly bonkers, but after that it felt like the episode became a generic Davies finale with the exception of that one thing...
I both love and hate the biregeneration. I heard people refer to it openly prior to this episode, so I was slightly annoyed that I was spoiled on that. But despite taking that surprise away, it is a curious twist...even if it feels like Davies having his cake and eating, too, all over again, just as he did with The Meta-Crisis Doctor. I get the point of doing it and allowing The Doctor to relax finally, but at the same time it still feels like a bit of a cheat. I'll have to sleep on this one.
Another reason I take issue to the biregeneration is that I thought Davies had finally done something that I've been hoping for years: A regeneration in the middle of the episode (although what I really want is a surprise, real regeneration in a middle of an episode but that'll never happen in this day and age...). Alas, that wasn't the case but at the same time, it's true. Maybe I'm being too nitpicky.
Either way, it was great seeing Ncuti Gatwa as The Fifteenth Doctor for more than a few moments before the crashing cliffhanger. I already love his energy and I cannot wait to see what he does with the role...and with a full wardrobe. Plus! His TARDIS has a jukebox! That'll be a lot of fun.
...which brings up another issue I'm on the fence on: Twinning the TARDIS. I wasn't really a fan of that with Clara and Me (mind you, that was a bit different) and I'm not sure how I feel about it here. Unless the means of twinning the TARDIS comes back to haunt The Doctor since the giant mallet evoked The Toymaker's chaos, right down to "winning the prize." Considering the mysterious hand claiming the dropped golden tooth (what is it with Davies and close-up shots of a woman's hand picking up a remaining fragment of a defeated foe as a tease of what's to come?), I feel like that might not be too much of a stretch.
Overall, I really enjoyed this episode but a rewatch in a few days will be in order to fully appreciate (or not) everything.