Saw it this weekend. It was good, but classic sequel-sickness. Tried to pack too much into the story, ended up giving a huge and complicated, but ultimately undeveloped, plot. They could have done a lot lot more with Khan, and with the internal politics of Starfleet. The fact that Jim Kirk goes from being a dropout to captain, then first mate and captain again, it's just too much suspension of disbelief in how lucky and privileged he is. Spock's rabid insistence on rules was utterly at odds with the simple humanity he protrayed in the first movie, and it rather irritated me. And lastly, the entire angle on Starfleet politics and how the main villain developed his plot was rather convoluted and annoying.
HOWEVER, I very much enjoyed the movie, and would gladly see it again.