"Well, here we go again!" -- Madame Vastra
[Warning: There may be spoilers here]
TL;DR: 6/10. This is Sloppy Moffat. Not at his worst, but still sloppy. It's saved by several excellent performances from the main cast.
A new Doctor's first episode does not have to be brilliant. In fact, quite a few of them have been utter rubbish over the years! Robot (Tom Baker), The Twin Dilemma (Colin Baker) and The Christmas Invasion (David Tennant) all come immediately to mind.
But they all have have one job to do, and they must do it well. You have to come out of the story believing that this new actor, no matter how different, is the same man as the previous actor and all the ones before them both. You have to really believe he's the Doctor when the credits roll.
That much, this episode definitely got right. Or maybe Peter Capaldi did despite the material. Don't really care which. When the credits rolled...I believed. I actually believed within about 30 seconds.
On the flipside, however, there's Clara. Clara was terrified and angry and completely out of control, and Jenna Coleman was brilliant at it. I now finally want believe in Clara, as a character.
Unfortunately, this improvement in Clara has a downside. In order to accomplish it, Moffat had to make it seem like she completely failed to digest her participation in the three previous stories. She knows he changes. She knows he's different every time. She has an advantage that many of his companions didn't the first time they experienced regeneration.
Let's accept that, however. Accept that, once again, Clara is being used as a plot device more than a person. She's the audience's POV character and for some in the audience, this will be their first experience of regeneration.
What follows works surprisingly well. Clara is a control freak who is now completely out of control. The man she knew and trusted has changed more than his face; she's not in her own time and doesn't quite believe she's going to get home again. She's scared, she's feeling betrayed--selfish but very human--and maybe a bit abandoned. Maybe she should get it, but she doesn't. And Coleman nails it.
Neve McIntosh's Madame Vastra is also at her best. She and the Doctor have always shared one key bond: they're aliens who not only spend most of their time amongst humans but kind of like them for some bizarre reason. Vastra is more obviously alien, but they both share similar impulses that allow Vastra to tell us the things the Doctor would never say.
In fact, of the "Paternoster Gang", the one I'm finding most tiresome is Strax. I've decided, in my own personal head-canon, that Strax actually knows exactly what's going on in perfect detail, all the time...and just likes screwing with people. I like that theory a lot better than Strax the Sontaran Buffoon. I'm waiting for the day he winks at a companion (and the audience) and proves it.
Of course, in the end, the ostensible plot of the story--and a great deal of its 80-minute screen time--isn't actually about any of that. The Doctor can never just recuperate. The place where he lands after his regeneration is always somewhere a crisis is taking place.
At first, we think the dinosaur he brought with him by accident is the crisis, but its not. Actually, the dinosaur is sort of a larger-scale metaphor for what the Doctor does all the time: drag people around space and time, get them into trouble, and sometimes--but not always--get them out again.
The real crisis is a bit of a dip into steampunk. We're in the Victorian era, and sure enough here are our old "friends", the 51st Century clockwork robots. Only these ones have been stranded for so long that they've done more than just replace ship parts with people parts; they've started rebuilding themselves that way as well. The result is sort of anti-Cyberman and successfully creepy.
In the midst of it all, the Doctor does something very...Seventh Doctor like. He leaves Clara in the midst of peril. Rather than doing everything he can to extricate her, he leaves her there, seemingly for terrible reasons. Whenever Seven did that to Ace, he was essentially manipulating Ace into actually solving part of the puzzle for him (even if it was just "how to keep the bad guy distracted"). Ace came to expect it, but for Clara, it's a new thing.
In short, by the end of the story, Clara has much more of a good reason for being uncertain of her old friend.
I'm trying not to overspoil things, so I won't talk about the plot's resolution, which is frankly a bit predictable. There's some nice ambiguity about it which I'm sure will play out over time, but the threat level by the end was just not very high.
Then, however, there's the coda, which I, like many other fans whose opinions I've already read, am not very happy about. The injection of Eleven into the story on any level was just a mistake. It says that Moffat (or someone else with decision making authority) is not quite sure of the decision they've made and they somehow need to resort to base sentiment and trickery to sell it. They don't. Capaldi had already sold it, lock stock and barrel, by that point.
It's five unnecessary minutes they could have cut in favour of just giving Clara 30 seconds or so to just...think about it; and then do what happens anyway. Talk it out with the Doctor. It shouldn't take Eleven begging for Clara to start accepting Twelve.
So there you have it. This was not a complete failure of an episode. It was not the complete hodgepodge mess that Time of the Doctor was, but it was still sloppy. In the end, its enjoyability lies entirely in the performances, not in the plot.
In the end, while a new Doctor's first story has to sell us on the character, it's the second story that sells us on his interaction with any old companions. So now, we wait and see what Into the Dalek brings...