I have 7,000 miles on my April deliv SMG 645
but don't like it . I would not say it is jerky, primarily because BMW has dumbed down the shift program to eliminate the jekiness. The tradeoff is that shifts in std manual mode are lazy and slow. Sport mode manual shifting is better, but can be a little jerky if proper throttle lift technique isn't used - and it takes a while to figure it out as it is not the same as a manual. I never, ever use the automatic D mode.
My advice is to purchase either the Step or the manual and pass on the SMG. This all said, I dislike it less than I did when I first got the car, so maybe in say, oh, 187,000 miles I might find it okay.