Loved the episode. A lot. It's now my favorite Christmas episode.
Part of it is because I adore 'is it reality?' plots and this one was well done. I know a lot of people are commenting here and around the internet that it's a ripoff (homage, if giving a positive spin) of Inception...but Inception didn't invent this plot. Any science fiction/fantasy fan of novels and shows since decades gone by could give other examples. So as long as DW executed the plot well, and they did, and added their own flavor to the mix, and they did, then is is a win for me. Plus the witty one liners, the well integrated Santa & elves, the lovely character moments, and that glorious sleighride thru London.
So this one goes on the 'nice' list, and maybe even, depending on rewatch, a more exclusive personal fave list.
I agree I would not have mourned if the show ended a couple minutes earlier. I'm ready for a new companion. It would have been a nice sendoff for Clara. But imo the episode did reset her relationship with Twelve so that they can interact with a different vibe next season and so that's a good thing.