The tl;dr version is that it was pretty good. It was certainly better than any of the last three years's offerings, but it doesn't rank among the best. I will certainly watch it again sometime, though I don't know when.
The only thing I knew about "The Return of Doctor Mysterio" was that it was a superhero story, and I was apprehensive. I mean, I've been a fan of superheroes for a very long time, ever since reading Spider-Man and The X-Men when I was a kid, but I know that the genre is very difficult to handle well, especially if you're trying to fit the superhero into an everyday world, which Doctor Who is - the world is non-powered except for the aliens and the Doctor, and in general, both aliens and the Doctor have very limited powers, usually low-level strength or limited application, or both. Throw an even moderate-level superhero into Doctor Who and you're going to overshadow the Doctor and throw off the balance of power in the show. This is the reason that Heroes imploded: they had a cast of characters each with one or two powers at most, and then Peter and Sylar, who were basically gods. This was the main mistake the designers made, and seasons 2 and 3 devolved into trying their hardest to reign those two characters in.
So, what does "The Return of Doctor Mysterio" do? It basically introduces Superman to the Doctor Who universe. Flight, super strength, invulnerability, and super speed. This wasn't a character; it was a trope. There's even an acronym for this: FISS for flight, invulnerability, strength, speed. (PS238 lampshaded this: in that world, this power set was so common in superheroes that rather than having superhero names, supers with this set were numbered.) Grant also had x-ray vision (but that was only included to make the adolescent "everyone's naked" joke; Moffat really loves to throw nudity into this show), possibly super hearing, and some way to shatter glass. I had had a fleeting dream that with the shots of both DC and Marvel comics, the superhero would be built with powers from different DC and Marvel heroes, but no, we got Superman, complete with the self-described mild-mannered alter ego. Maybe Moffat thought that he had to go with Superman because no one knows Marvel enough to recognize the nods to it. (That's sarcasm, by the way.) In the end, the balance of power had to be maintained by having the hero arbitrarily decide to lay aside his cape - very poorly handled.
Anyway, that's my take on it as a superhero fan. Sloppily done and kind of a terrible tribute to the genre.
From the point of view of a Doctor Who fan, as much as I can without the superhero coloring anyway, the episode was good enough. I loved the opening, showing the superhero genesis in a very nice, compact scene with great Doctorage, and it bolstered my anticipation for the rest of the show. The alien invasion was effective enough but not really compelling. Yes, they needed to be stopped, but they just didn't feel like much of a threat. It was pretty obvious that they were a secondary plot point, and that the real story here was the relationship between Grant and... I can't even remember her name. Fletcher. That was her last name, so I'll go with that.
But that probably tells you how interesting that part of the episode was. (It wasn't Jessica, was it? Because isn't Jessica Fletcher from Murder She Wrote?) It was the same old same old: alter ego loves the girl but won't tell her who he is; girl friendzones alter ego; girl loves the superhero and can't figure his alter ego; world is threatened and superhero unmasks to save it; girl realizes they're one and the same; boy and girl hook up. (Wait. This was the plot of my last long fanfic. Dammit. I'm such a hypocrite.) The plot was banal and there wasn't any chemistry between the boy and the girl. Fletcher, in particular, was obnoxious - I couldn't stand the scene where she grilled the Doctor using that stupid Bill-shaped toy, and she was written like Moffat thinks that an intelligent, strong woman only speaks in quotable sound bites. (Well, we already knew that.) Who in the world, when realizing that she loves her best friend, cites the reason she loves him to be "He's never lied to me"??
The Doctor part of the show, which felt like the smaller part, was a lot of fun, and I enjoyed his interactions with Nardole. I'm not that fond of the slash-headed guys, if only because they seem so ineffective. They're lucky that their adversaries are never armed, because it takes them way too long for them to get to their weapons. I'd love to see them come up against Leela.
I will say that the final resolution was stupid and anticlimactic. Grant had no idea that the ship was a bomb or that a sudden impact that would set it off, so catching it carefully on the ground was a stupid choice. One little miscalculation, or an overestimation of his ability to stop a multi-ton falling object (or a timer on the bomb!) would have been catastrophic (not to mention, shouldn't that hull be HOT). But ignoring the bad science and strategy, that scene demonstrated a lack of understanding of what a superhero show's audience wants to see. We want to see the superhero doing superheroic things: telling the girl, "Sorry, but I have to save the world now" and flying up and meeting that ship head on, and throwing it into the sun (love that nod to the Sentry, though I'm sure that was completely inadvertent and unintentional). At the very least, we want to see him catch the ship. You can make up your own appropriate metaphor for building up the tension, the danger, and the hero's resolve to the breaking point, and then panning the camera away from the action and showing us scenes of calm silence for fifteen seconds.
But, ignoring these problems (and I will readily admit that many of them stem from my love of superhero stories and how this episode did not live up to that genre), it was a fun enough adventure and I enjoyed watching it. It's probably worth watching simply for the Doctor and for Nardole, though scenes with either of them were few and far between.
Oh, I forgot, one last thing that I wanted to mention. I don't know anything about direction or cinematography, but for an episode set in the largest city on the planet, it really felt like there was nothing beyond the edges of the screen. It actually felt more like a recording of a play than a TV show to me. Even the scenes set on the Manhattan rooftops felt like they were isolated in some alternate dimension. I've really missed the feeling of reality in Doctor Who episodes set on Earth for a long time now, but this one was over the top. Was this filmed on location in New York? Because if so, they wasted their money.
Addendum: As I mulled over the plot, I figured out what I would have altered to make this episode a lot more meaningful. I would have started with Grant being a teenager with a crush on Lucy (I finally figured out her name) when the Doctor first visited him and he got his powers. Then, at the end of the episode, I would have the Doctor figure out that the stone hadn't given him superpowers as "what he wanted", but as "what he thinks he needs to attract her", and thus, Grant learns that he doesn't need to be a superhero to attract Lucy; he just needs to be himself. Then, when he finally learns that at the end, his powers leave, because he doesn't need them anymore. I don't know. To me, that makes a better emotional impact, and a much better Christmas-y story, than the simple superhero/alter-ego story we got. Of course, I could be daft. I can never tell, from the writer's viewpoint, what makes a good story, and it makes my hobby as a fanfic writer quite a bit of hell. :/