I brought this up for a couple of reasons.
#1 What kind of weight would this add?
#2 If you used it on a roof can it be painted or does it come in other colors or can you get a smooth finish?
#3 If you use it as undercoat will it actually stick hard enough to stay on?
I would not use it for the sides as I think it would look cheesy.
However I had a friend of mine go around the entire bottom eight inches (like an 8" stripe) of his Ford pickup, it looked pretty good.
I can tell you that after 7/8 years that he has had it on, there is no rust around the bottom of the doors, wheel wells, or anywhere else.
That is the only reason I considered this product.
Here is a van roof they show on their web site:
On his truck it looks pretty cool because it is more of an accent than a big black area sticking out like a sore thumb...