Cumberbatch sold me with the "you should have let me sleep" scene. Cold and dispassionate one minute, raging psychopath the next.
I thought that was his best line. I didn't like where they went with the character - he was ambiguous and could've been an uneasy ally, but then the writers got cold feet and made Kirk shoot him, and he went all bananas. You know, so there wouldn't be any grey areas, to make it easy for the audience: "good guy vs. bad guy".
That was a bit of a downer, because I was quite pleased up until that point. They could've saved his bigger bad status for the next movie, but they decided to cop out. I don't like it when they cop out.
Like Doug and Rob Walker said: the movie was cool up until the last half hour, where it went sort of downhill. Not enough to ruin it for me, but that turn it took annoyed me nonetheless.