I have a 330ci with SMG I and I really like it. There is no problem with smoothness once you get the hang of it. I'm sure that after just one day, everybody who knows hoe to drive a stick, can learn how to shift at least as smoothly as with a regular tranny. The auto mode I admit is far from satisfying, but then if you are interested in an auto transmission, you want a sliptronic anyway.
I have not tried a SMG II, but from what I have heard, you are able to have a lot more control of shift speeds. The SMG I has two manual modes, sports and regular. In sports mode, the shifts are faster but less smooth then in regular mode. The SMG II has six different programs, which basically vary how fast your gears are changed, and apparently it has the ability to adjust to your style of driving. It also has additional features such as up-hill start, IMO for drivers who are not familiar with operating the e-brake.
In short, I love the SMG I and have no regrets about getting it. If the SMG II is even better, then I feel comfortable recommending it, even though i have never driven one.