I ran an ACT 6-puck clutch and Fidanza 15lb flywheel in both my G35 and 370Z (both were daily drivers...aka 10-20k miles a year). It never slipped, which was most important to me. The one thing I hated was the consistency of the "grab", but this could have been due to a bad master/slave cylinder.
Yes, the 6-puck is heavier. Yes, at times the clutch would be either in/out with no play. The question is would I buy again? Yes, I would. Don't let it scare you
