I enjoyed the Christmas special. I actually found it to be the best episode of Capaldi's run. It was fun, the characters were endearing, stakes felt high and there was a nice emotional subplot with River that gave the Doctor some growth without being too on the nose. In fact, the Doctor was actually likable in this episode. He was crotchety without being annoying. He was quirky without trying too hard (which is how it feels with the sunglasses and guitar). Even the sushi gag and him constantly eating worked well, because unlike the sunglasses, guitar, and other quirks that the tried to give 12, there was no bit that basically felt like the writers saying "Look at how quirky he is!" Instead it was subtle. I feel like this was the best written portrayal of Capaldi's Doctor and I hope now that the writers have found the right balance for him it continues into the next season.
Matt Lucas's addition did not annoy me as much as I thought. In fact, I look forward to him as a companion now more than I do Bill (whose voice is simply atrocious). I'd honestly be okay with a season of the Doctor and Lucas's character just going on adventures without a regular companion. I think that would be far more satisfying than Bill, who seems to have nothing extraordinary about her. She feels like an interchangeable, generic, companion.