I just installed some Manlys over the weekend and have put about 3 miles on them.
Holy crap, I think I turned my car into one of those paint mixers at Home Depot. My steering wheel is a blur at idle, it's shaking so violently. At idle (1000-1100 rpm) every body panel seems to resonate. It's actually louder in the car at idle than it was previously on the freeway. Above 2000 rpm, the vibration goes down substantially, but doesn't totally disappear.
Shifting and throttle response feel no different at all. What is different though, is how the power feels transmitted through the clutch. As soon as the clutch begins to grab, the car begins to move. It's hard to describe, but the car feels much more connected as you use the clutch.
I'm very tolerant of un-street worthy cars, so I'm going to live with them for now, and just go back and pick up any parts that fall off. The vibration makes the car feel that much more raw. I know it's completely in my head, but the car actually feels faster now. If you're looking for imaginary horsepower, this is probably more cost effective than an air box or exhaust.