Lighter wheels definitely improves the acceleration, braking, handling and overall feel of the car. The wheels respond faster to bumps on the road too so it can improve ride quality as well. Lots of people will ask for numbers to put behind it... laptimes, g-forces, acceleration numbers. The difference in those are sometimes small but the the "feel" of the car is just that much better. Kind of hard to explain.
Modified magazine tested lighter and heavier wheels years ago in their wheel test to see what kind of effect lighter wheels had. The lighter wheels are definitely faster but not definitively so.