So having just shoveled a yard of sand out of the back of the truck (one of I guess a hundred or so of sand, gravel, etc. in the last eight years), I took the time to examine my Line-X coating. I have to give this stuff two thumbs up and five stars. Besides the dirt loads, I've hauled roofing, lumber with nails sticking out everywhere, scrap metal, all kinds of stuff that would scrape the hell out of the metal truck bed.
The Line-X has scratches in it, and a couple of light gouges, but zero penetration of the coating. One of the reasons I looked today was all the times the shovel blade made contact with the coating when I was working around edges of the bed. I hit the stuff hard, and the shovel does nothing to it. I think my gouges are from the sharp corner of some heavy metal stuff I hauled a while back. Otherwise the stuff is freakin' nearly indestructible.
Anyway, if anyone is ever thinking about a bedliner, I highly recommend Line-X. It's definitely going into my next truck.