This episode is definately my favourite christmas special episode. It included Catherine Tate who is my favourite comedienne. Donna is a classic character and I'm glad they brought her back in the 4th series. The Racnoss was a marvellous enemy, a giant spider, genius! At first I thought it was CGI at first, but Sarah Parish pulled off a brilliant performence. The Christmas episodes are always exciting, apart from Voyage of the Damned. I think this episode set things up for later seasons.
The idea of the story was very good, and you didn't see the surprise twist coming, you really didn't know that Lance was in on it.