Had it done to my last truck, worked well but it did fade with time, going from black to a greyish black color. May want to think about a contrasting color due to this, mine was red with black rockers.. Also unless they do a good job scuffing the paint beforehand it may start to peel off on the bottom, however that didn't start to happen for eight years.
Also I drove the truck for ten years in a area where heavy salt is used (Nebraska) and did not have any rust issues with the truck, I did make a point of spraying off the underside after every snow storm though.
This time round I went with the clear paint protecting film, not enough time to be able to say how well it works yet.
John K