No problems to report with the nismo wing, nissan claims 150lbs of downforce but that hasen't been proven one way or the other, that might just be how much it weighs.
IMO wings are more of an athestetic thing on production cars, no one really needs one even if you do track the car. I do wish the nismo wing was a little smaller though. I think the Z looks a little silly without at least the sport package spoiler on the back end. Just my $.02