I suspect that MIB does have some level of trance/mind control, etc when people are in his presence. He keeps his followers because he threatens to take away what they want most. Now that Claire has some distance, I predict she will come around. Sayid's not a 100% gonner, so he could have been persuaded by Desmond. Sayid may have shot next to Desmond or maybe in the leg, in order to sound a gun shot for MIB.
Sawyer seemed to be pro-MIB until more recently now that he's spending less time with him; and has seen both sides of the war. Sawyer is also less needed to MIB, so Sawyer probably feels tossed aside when he recruited Sayid, and now Jack. He believes that MIB is a liar and does not trust MIB's promises. Therefore, he can snap out of the mind control MIB had over him. Also notice that MIB directly addressed Jack, and no one else. Therefore, to go with the tension between Jack and Sawyer, Jack seems to be trumping Sawyer again.