Vibrations that come through the transmission are damped by the dual mass flywheel which can (not sure Nissan did) be tweaked to dial out specific frequencies of noises/rattles. I think Audi do this with most of their cars.
I'd really suggest that you try out a car with one if you are unsure. Many dont find it a problem, but mine just felt a bit broken at lower revs (<2.5k) which being quite a lazy driver when I have passengers, it made you quite aware I had a SM flywheel. The pickup, while better, wasnt noticable during day to day driving, but the rattle most definately was.
Horses for courses and all that. Plenty find it makes a nice difference and the sound doesnt bother them, and plenty of people find it doesnt make much difference to performance but the sound is annoying. I was kind of in between and I know there are other less vocal members out there that feel the same, hence why I'd suggest getting in a car with one to see what its like