Is this thing worth it? I would think normally there is not much air going under the car even with the stock splitter in front, so what's the use? Also to those who have installed one, are there bottoming out problems?
Air is a fluid. Yes there is still lots of air underneath the car, heck there is lots of air under an F1 which is why they design the underside of the car to take advantage of the movement of this air.
The splitter will do nothing to prevent air from going under the car. If anything it acts more like an aerodynamic surface to help air remain laminar (as opposed to burbling) which reduces drag.
This air scoop is a rather crude method of grabbing the air from under the car and passing more of it along the cooling fins of the radiator and is only as efficient as the velocity of the air and outside air temperature.
I think an air to oil transmission cooler would be a better solution, but I don't know if anyone makes one for a manual. If not the scoop is better than nothing but I'd prefer one shaped a little more like a NACA duct.