The script idea including Khan was tossed to JJ Abrams during the filming of Star Trek. A deleted post-credits scene was going to show the Botany Bay floating in space. But JJ pulled that in case he decided against a Khan story. The reason the movie was delayed a year was because of their uncertainty over using the Khan script. Abrams was well aware if they fuck it up, they lose the fans.
They looked extensively for someone they felt could carry the Khan flag, and settled on Benecio Del Toro. But Del Toro dropped the project and it went into hiatus for four months. They pretty much screen tested every Latino actor in Hollywood, and several non-Latinos they felt might carry the project, and Abrams was ready to toss the entire script. Then Benedict Cumberbatch walked in.
Cumberbatch is a life-long Star Trek fan and his work in Sherlock on the BBC has gotten him international acclaim. He's in huge movies, such as Tinker Tailor Soldier Spy, War Horse, and The Hobbit in a lynchpin role as Smaug, so he's definitely shooting up. Apparently he gave a 1/2 hour unprompted audition and JJ was convinced that he could be Khan.
I trust JJ and I think Cumberbatch is a great actor. He'll do to Khan what Chris Pine did to Kirk - strip away the things we knew about his mannerisms and leave the core of the character. He won't be impersonating Ricardo Montalban, which I think is a good tack to take. I could be wrong, and I remain open to that possibility, but based on the tweets during filming from Chris Pine, Zach Quinto, and Simon Pegg, the principle actors all got along really well on set (culminating in an unscheduled midnight party in San Francisco with those three and Cumberbatch) and I always think that is a damned good sign for a movie. It means more chemistry in the characters.
It comes down to whether or not you trusted JJ Abrams as he destroyed Vulcan, killed Kirk's father, brought Spock back in time, and put Christopher Pike in a regular wheelchair (beep beep). I think he's earned my trust, and I will once again be first in line for a Star Trek movie he's made.