I quite enjoyed this special, easily Davies' best Christmas episode (albeit that's a low bar for me). There wasn't anything that really made me cringe, aside from the Goblin King (yuck).
I really liked the idea of the sky-faring, time-traveling, pirate goblins who feed on coincidences, chances, and bad luck. They're such a ridiculous notion that they fit in perfectly with Doctor Who, especially Davies' take on the show. I will enjoy seeing them again, whether or not they're connected to Ruby's larger story.
Speaking of whom, I love Ruby. I initially worried she would be another Rose but she is indeed quite different beyond the surface. Putting aside yet-another-mystery-companion trope, I love Ruby's story, how she fits into a larger family of foster children, and how she was the one who stuck around. I adore Carla and Cherry and I look forward to seeing more of them (which, of course, we will since this is Davies after all).
As for the mystery itself, I'm mildly curious about it. However, after so many companions tied into bigger mysteries, and I know I've been saying this for nearly two decades, I long for the days when The Doctor simply runs into a companion and they have grand adventures in all of time and space without a giant axe hanging over them. To be clear, I feel that this is the fault of all of the modern showrunners, although I think Chibnall did handle this the best...I just wish Yaz had been better utilized.
For all of the fanfare and "buzz" I saw in this forum about both the goblin song and the new sonic, I'm largely indifferent to both. The only thing that really stood out to me about the song was how the singer's voice just sounded like another human without any attempt to make her sound like a goblin (or rather non-human). And the sonic is still a magic wand even if it looks like a garage door opener now.
Spring can't come soon enough!