My pipes are the lowest part of my car. I have H-techs and bilsteins, so a slight drop. I bumped into a curb with my pipes once and there is a small dent, but nothing that affects functionality. Other than that, rocks and debris have left some scrapes on the pipes, but again nothing that concerns me at all. I am very cautious when I'm straddeling (sp?) large items though.
I got the living hell scared out of me a couple of weeks ago. I went to an impound yard to pick up the car and as I was walking up, I saw them moving my car around off in the distance. They were moving it with a very large forklift. The forks went straight under teh front of the car. I thought my FMIC was completely toast. When I finally got that car back I looked and there wasn't a single dent or scratch on those pipes... to this day I still think it was an act of God that saved them.