I have the front one on only right now.
Especially since it's broken in, the vibration is much less. You can feel the engine more through the steering wheel now, which I like, and the car only really noticeably vibrates under 1000rpm.
Wheel hop was reduced by 60-70%. After it broke in some of it seemed to come back so I can't wait to put in the rear one too - My goal is to TERMINATE wheel hop!!
(guns)Wheel hop(gun)