SPOILER LEVEL: High
Wow. A great swan song for the 10th Doctor. A clever way to bring back the Master and have it still be John Simms, major stuff with the Time Lords, and a wonderful closing retrospective of the 10th Doctor's entire run. I don't think any Doctor has ever gotten such a big send-off.
The only part that didn't really work for me was the big climax of the Doctor trying to decide if he shoot the Master or Rassilon (!!!) using Wilf's normal old human gun. I didn't much see the point-- either one could just regenerate, and that's assuming they were normal Time Lords. As it is, they were Super-Master and freakin' Rassilon. (And speaking of the Super-Master, it bugs me when they bring in new powers out of the blue, but since it was obviously meant to be a side effect of the Master's resurrection going wrong, I could handle it.)
I LOVED getting a taste of the Time Lords' part in the Time War, and the image of Gallifrey itself filling Earth's sky. Speculating on who Mysterious Time Lady was with everyone was also a lot of fun.
Farewell, David Tennant. It was a great run. Thanks for staying around for four years worth of stories.
And last but definitely not least, major kudos to BBC America for airing them here in the US just 1 day after they aired in the UK!