I used the $100 Herculiner kit purchased from my local Tractor Supply to line the inside of my Jeep after I got tired of cleaning mud out of the carpet.
Install was relatively simple. Everything was removed from the interior, then I sanded/scuffed all the metal that was going to be coated, wiped it down with acetone, masked off want I didn't want coated, and then gooped the stuff on. Actually applying the stuff was the easist part, most of the install time is going to be the prep work. How well you prep the surface is going to determine how well it sticks.
You MUST wear gloves, long sleeves, and pants when dealing with the stuff, and the clothes need to be ones you won't care about throwing away afterwards. The liner material can't be gotten out of clothing, and will take around two weeks to come off your skin. There was a post floating around some of the 4wd boards awhile back from a guy who stopped to take a piss while applying the stuff and got it all over his dick. [shock]
I coated the inside of my Jeep almost three years ago, and it has held up pretty well. Overall I like it, but I don't know if the coating the DIY kits give would be thick enough to stand up to the kind of beating a truck bed is going to take. I'm not throwing/sliding concrete blocks, lumber, etc, etc in and out of the back of my Jeep on a regular basis.
If you decide to go with the DIY kit make sure you get the Herculiner or Durabak kits. From what I've heard the others, like Duplicolor, are just paint.