i happen to have those stick ons on my hood and they are functional as well, did it myself. as for expensive, with paint that i did myself it only cost about $200 with installation of the scoops cuz i didnt know how to put em on exactly. and youre right, it is only a matter of drilling and cutting, tin snips was actually my tool of choice, and it worked wonderfully too, not too pretty but it got the job done. as for the coming off, the only way i could see them coming off is if you drilled the holes in the scoops without cutting the holes in the hood allowing for the air to go somewhere. i did happen to notice a small section that seemed to be coming off but with the lifetime garauntee that says they'll stay on as long as you own the car, i took my chances.
heres a pic but with the shadow of the tree, its kinda difficult to see em and its not too good of an angle either.