Well, that was surprising. And a big step up from the last few episodes. Though I still do have a few quibbles here and there.
Though I suppose the MU tell was right in the title, I still didn't quite expect this. More than that, I was surprised that after Georgiou passes through the doorway, the show stopped cutting back to the real Discovery. It certainly made narrative sense to do so, given the episode was framed from Georgiou's POV. It also made casting sense, because basically the entire main cast can may their MU counterparts.
I thought the episode was very effective for what it was. Obviously the MU is a good excuse for grimdark camp, and we saw that on display here. But there was a surprising level of depth in terms of Georgiou's character here as well. We get to see that even though she's way too comically evil to fit in with the Discovery crew, she's been changed by her experience there to the point she can't just slide comfortably back into her old role. SMG was incredibly effective as MU Michael. In contrast, Mary Wisemen disappointed as Killy. She was just...there. Not adorkable like Tilly, but not campy and evil either.
My main issue with this episode though is in a serialized show with a limited number of episodes this seems...kind of self-indulgent? They are using a two-parter to get Georgiou irrevocably off the show, but by nature of the story they're telling now nothing that happens in the MU will impact the PU characters at all. As far as the Discovery crew are concerned, the relevance of Georgiou ended when she passed through that door. In episodic Trek this would be fine, but here I don't get why you do it when you can write Georgiou out of the series with more economy. And while I enjoyed this episode, they're really just retreading Season 1 plot points (albeit in a better way than was done in Season 1). I think Georgiou going to the 32nd century of the MU would have been more interesting, but undoubtedly it would have been harder to deal with from a casting and set perspective.