I've had one of those cheapo $39 or so ABS Camaro re-pop spoilers on my 74 Road Runner for several months now and it's held up great. My plan was to see how well the ABS would fit and last before spending money on a steel one, but the plastic one has held up just fine. No warping, cracks, or other defects.
I mounted mine at a different angle than the ones on the other cars in this string. Mine is further forward and at a shallower angle.
What I have found very interesting is this spoiler does alter the aerodynamics of the car. I would get in my car and usually not really look at the rearview mirror until I got onto the Buckmann Bridge and had to cross over two lanes of traffic. That's when I always noticed my mirror was a little off and I would adjust it to where I could see the whole rear window. Then at lunch I would get in the car and look in the mirror to back up and see it was off again. Same seat, same mirror, same me, so why was I always having to adjust the mirror? Then it hit me that I was setting the mirror while stopped, and when doing 70 or so on the bridge, and the angle of attack of the car was being changed by the spoiler. Not a lot, but enough to throw the mirror off a bit.