Those covers, from what I gather, do 3 things.
1. slight improvement in MPG by providing a smoother airflow beneath the vehicle. Germans have been doing this for a long, long time.
2. Bottoms of engines/trans stay much cleaner. I like this, a lot. I have had 2 S60s, one with the cover, one without. The one without is filthy beneath, whereas the covered one stays much cleaner. (I still take care of one of them).
3. noise dampening
I keep the covers in play. Especially with the F150 which requires no jacking for the O/C, and small electric drill with a socket on it makes the felt diaper a no-brainer.