(Some minor spoilers follow)
In all honesty, I would love to tell you that this year's Christmas special is a return to form for Russell T. Davies after the debacle of Journey's End. In fact, I can honestly say that 85% of The Next Doctor is nearly pitch-perfect, mixing equal parts pathos, comedy, and with a really creative premise that I doubt the original series proposed (unless there has been a Cyberman story set in the past that I've never seen)...in fact, if the sequence that happens 25 - 30 minutes in doesn't break your heart, it means either you're jaded, and/or you have no heart.
Our story begins in 1851 London, when the Doctor comes across a young lady named Rosita, who is in danger, and who is yelling for "The Doctor." However, much to his surprise, another gentleman - who calls himself "The Doctor" (and who we'll call "Doctor Next") arrives to help save the day. Thanks to a slight turn of events, our Doctor comes to believe that Doctor Next is his next regeneration. Of course, there is a complication with the Cyberman attempting to take over Victorian London...
...and most of the episode works beautifully. David Morrisey hits the right tone - almost as if he were next in line to take over from David Tennant. (No, I'm not speculating at all. I'm letting the rumors fly freely) It's a great takeoff on a classic Victorian adventurer, and Morrisey handles the role with aplomb, even with the script complications. For most of this episode, it's an almost pitch-perfect mix, probably the best since The Christmas Invasion (and definitely much stronger than last year's Voyage of the Damned).
However, and I'm going to try to talk about this without spoiling - the Cybermen's "big bad" is simply lame and unoriginal. It seemed more like Davies was trying to go over-the-top in light of Journey's End - I just was unconvinced. It seemed done more out of a sense of "let's-give-the-effects-guys-a-challenge" than a legitimate threat. But making it through that sequence leads to a touching, totally heartbreaking sequence...but one that suggests that this Doctor's journey will be coming to an end, and almost reads as if the Tenth Doctor has been - on some level - paying for the Ninth Doctor's sins in many ways.
If you get a chance to see it, do so. Just don't mind the wonky bits 47 minutes in.