I thought the idea was good (although obviously a bit Inception meets Alien) but was executed a bit messily.
My problem with recent episodes is Moffat always tries to shoehorn in too many ideas, whilst I think a simple and well-executed plot is better. In this one Santa was the extra idea shoehorned in. In the last episode we had the Mistress, we had the magic Cyber rain, we had the land of the dead, we had UNIT, we had Danny Pink becoming a good Cyberman...it was just so much stuff it prevented an elegant plot.
If we think back to the episodes generally considered to be the best, we have Blink - one, single idea for one episode, executed wonderfully. Or Human Nature, similarly not too crowded with introductions of new things. Then think of some of the worst ones - the final Donna one, for example, where planets are being moved, people forgetting, Dalek king comes back, Torchwood comes back and does nothing, Martha comes back and does nothing, Rose comes back and does nothing, the doctor gets a twin - too much.
I also thought the old Clara ending was a perfect ending - it was a little 'Peter Pan comes back to find Wendy's grown up' kind of thing. It was a bit of a shame to find that to just be a decoy.