One word... EXCELLENT.
This episode featured everything Doctor Who is known for, fantastic sci fi concepts, out of this world creatures, bundle of laughs

, and most importantly, some genuinely emotional moments.
Truth be told, I still preferred last years Christmas special over this. It's just not the same without Billie Piper as Rose, though Catherine Tate as Donna aquits herself well. I can understand though why people find her annoying, but I find her absolutely hysterical here.

Great interactions with Tennant. Also, the last Christmas special had more of a sense of dread, whereas the tone here is a bit more tongue of cheek (not that I'm complaning

).
And while I still prefer Christoper Eccleston as the Ninth Doctor, there's no denying how brilliant David Tennant is as the Tenth. And he's especially great here, showing bits and pieces of the Doctor's sense of loneliness after losing Rose. It's also a kick to see him bring out a darker side of the Doctor's especially in the scene SPOILERS
where he drowns the Arachnos queen's children (but did he really need to empty the Thames to do that?) 
SPOILERS.
So anyway, brilliant way to start the new season, and I look forward to the coming episodes. Especially since that preview in the end had me salivating!