I wasn't going to write a review because a lot of people already said most of what I think, and more eloquently than I ever could. However, I want to point out that one of the things that really annoyed me was how this was a bad episode of TV. I love Doctor Who, even when the plot is not all that; hell, I adore the Movie, which we know is not really good. The Reality War failed in three very, very important things an episode of a show, let alone a season finale, could not have failed at:

1) It was impossible for the audience to know where the plot was going, because the goal posts kept moving and completely new things kept being introduced at breakneck speed. The Deus Ex Machina of Anita's literal magic door key is an insane offender. How could we know Poppy was 'impossible', why does the Rani need Omega specifically? Because the surviving Time Lords are sterile now, we're informed in an exposition dump. Why is Omega a giant mummy? Because he became his own legend in the Underverse, we're informed in an exposition dump. What are the bone things? They are Underverse creatures, we're informed in an exposition dump. How will we protect Poppy from the wish being undone? Susan Triad will build a Zero Room in... what, 15 minutes? It was all insanely clunky. The vindicator is a laser gun because sure, why not. The Doctor has to die because, you see, he needs to shoot regeneration energy into the vortex. HOW were we supposed to put the pieces together and follow the story if we didn't know basically ANY of the rules? If your audience can't even begin to grasp at what the rules of your story are, the solutions are not going to be satisfying. They need to be laid down, but not in a verbose, technobabbly exposition the second before it becomes plot relevant!

2) Not a single character arc is realized. Belinda has her whole life rewritten, is now not even the same person as the Belinda we met in 'The Robot Revolution'. Ruby also has no arc; she was there for the story and did things (which more than I can say for Belinda), but she did not grow or learn anything. What she got was trust issues and PTSD. The UNIT gang are there, sure. Shirley was saying she was gonna bring down god last episode, but she just ends up being one of the 23489 characters in the UNIT tower. Same with Mel. Kate was given what has to be one of the cheesiest, most forced lines I've ever heard, and then spends the rest of her time spinning the tower around. Anita suddenly had a crush on the Doctor all along, is now pregnant, holds a door open to [checks exposition notes] real time to keep flowing. Rose Noble is there for 0.3 seconds. Everyone keeps hugging and telling each other 'you're the best!' and these characters just do not feel like real people. The Rani gets eaten alive by Omega. There is no character arc for anyone. The closest we get is with the Doctor himself, who had a bit of theme of looking for family going on in this incarnation.

3) It doesn't know who its audience is. There's things here for kids, there's things for regular scifi fans, and there are things for superfans. The problem these aspects are not playing together to make this an episode many different people can enjoy: they are playing against each other. The kiddy bits feel more embarrassing than fun, the fanservice to Classic Who is confusing, makes the plot needlessly contrived, and the scifi has wand-wavy, arbitrary rules. The pacing is also all over the place, with some fast action suddenly followed by strange, long scenes of 10+ characters standing around listening to long winded explanations.

I'm gonna stop now. It's been almost a day I just keep getting more and more frustrated with this episode. What a damn shame.

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ETA: I don't wanna be all downer here! Here's some assorted things I liked: Anita's 'I'm just the hotel manager' jokes; Kate's disgusted 'I'm wearing tweed'; Ruby fighting to get people to believe her about Poppy; all the set designs and special effects (yes, even mummy Omega); Ncuti's lovely, tender acting in the quiet scene where Belinda is 'reminding' him of things she 'told' him (gods, I wish we had more time with him). And Billie? I'm optimistic :)