I'm running "truck bed paint" in the rear wheel wells of my 04 Sierra. I put it in about 1.5 years ago and it's holding up quite well, think I have one small nick in it so far. I think I'd do this again over Line-X'ing them as it is fairly smooth and thus wont hold dirt/mud like Line-X will can with its texture. My in-transit Raptor is already black though, so I won't be doing anything to it.
I'm a huge fan of Line-X though, have had it on pretty much every truck I've owned in the past 10 years and the Raptor will be getting it as well...but only in the bed.