THERE BE SPOILERS AFTER THIS! YOU'VE BEEN WARNED!
Holy crap, what a mixed-bag episode. In typical RTD-fashion, we get an episode that has a great overarching ("big") story and mediocre to bad single ("small") scenes. But all that is forgotten in light of the fantastic ending. I loved the dilemma the Doctor faced and I especially loved how his breaking of the rules for the right reasons made him "evil." I was so hoping they would have her commit suicide to put things right and I was not disappointed - it was very well done and extremely powerful. "Timelord Victorious" (akin to the Master's "Timelord Vainglorious") is a brilliantly perfect way to end Tennant's stint as the Doctor. The mind reels with the possibilities ... I would especially love if the Christmas Specials had the Master return in order to stop the Doctor who went "off the reservation" and became the bad guy. The role reversal would be ironically awesome. As usual, I can't. Fucking. Wait. For the next Special.
P.S.: I still can't grasp the awesomeness that was the last 5 minutes. Holy cow! Especially the bell-tolling of the Tardis, signaling the universe is in grave danger, while the Doctor decides ... what? What did the last "no" mean? "No, I wasn't wrong and I am not evil?" or "No, I won't succumb to the evil side!"?
P.P.S.: On Second viewing, anyone notice how eerily significant the Doctor's line about the robot at the end was? "He's lost his signal. Doesn't know where he is."
"We're on the side of the demons, Chief. We're evil men in the gardens of paradise, sent by the forces of death to spread devastation and destruction wherever we go."
-- Colonel Tigh