SMG II is even smoother and quicker than the Magnetti Marelli system working in Ferrari cars. It's a serious piece of F-1 technology on the road.
Nevertheless, I'd get my M3 with a standard six-speed manual transmission, any day.
I always had the theory that the only reason car companies start offering these transmissions on their premium sports cars (F355, F360, Vanquish, M3...) is that they think a lot of the market is rich fruitcakes who're afraid of learning how to row some gears, and want to appeal to them by offering transmissions with automatic modes, like SMG and SMG II. It just seems like too much tech for me.