Wow. It amazes me that I totally disagree with most of you. When the rumours first started to circulate about the possible use of Khan as the villian I was really disappointed. I wanted my characters to encounter new situations, not retread the past.
However, Cumberbatch's performance totally won me over. Where TOS "told" me that Khan was a huge menace and an imminent threat, Cumberbatch showed me that this was the case. I believed that he was capable of anything. I didn't have a feeling of dread when Montalban was on screen...I certainly did when Cumberbatch was.
As for backstory...I didn't feel it was necessary in this case. We were shown that Khan was concerned for his own people, and that he believed that they had all been wronged. He was going to act to correct the wrong. I didn''t need to know the whole story. He and his people could have been evil just because that was their nature...and I find that even scarier than than a full explaination.
For me, this Khan rocked.
This... I was hesitant about it but that scene with the Klingons attacking and him... that was a "holy **** do not mess with this guy..." and it only got more and more intense. Not only did he still manage to be charming and likeable at times, but there was always an air of menace around him that Montelban never really had... just a crackling undercurrent that put everything on edge til it started to explode. The scene with Marcus is just... that is a classic moment to me.
And now, for however long these movies continue, there will be that nagging worry that he and 72 like him will escape and Kirk will have hell to pay. If that becomes a movie or whatever... well, it might make TWoK look like a nice drink at a bar. Cumberbatch took Khan and did something I didn't think could be done... he made Khan absolutely terrifying. Cunning and dangerous was always there.. but now he is a truly frightening figure in the Trek universe.
And his role actually makes even moments regarding genetic manipulation that much more resonate and believable. You go "yeah... we don't want another Khan. Totally get these restrictions."