A bit late to the thread, but I really enjoyed it too.
To start with I was not too sure, the humour seemed to come out of nowhere and I was not expecting it, especially after the end of the main run of episodes. But I quickly settled into it and enjoyed the romp. It seemed as though everyone taking part was enjoying it as well which really helped get the episode of the ground.
It was wonderful seeing the Doctor's devilish side as he relished in the idea of River not knowing who he was.
As Mr Orange said it was clever how it moved from the comedy to seriousness as it rapidly became apparent that this was just before River's cycle would be complete and we were back at the beginning/end for her character.
I particularly liked the reference to the diary. Nearly full up. and given to her by someone who knew exactly how long her diary needed to be.
So a lot to recommend the episode, just little touches like the Doctor giving her the screwdriver, or River not realising that he had another set of regenerations. It was nice to see not as all knowing as she has been, and the Doctor being the one with the 'spoilers'