In my opinion, unless you live in a warm dry area away from salt air, road salt, etc., "under coating" your vehicle is worth it's weight in gold. Not only does it keep panels from rusting, but it keeps the electrical wires, connections, bolts and nuts - everything from corroding. Even if you don't live in an area where they use salt or a de-icing agent on roads, and you do any off roading at all, it will be a benefit. When you "play" in mud, it gets inside the panels between the inner and outer shiny painted side and continues to hold moisture unless everytime you go in mud, or on a wet dirt road, you spray everything out with a good stream of water. This means up inside the wheelwell and between the two panels on each boxside, opening the hood and spraying inside the fenders between the inner and outer panels, a little spray inside the doors through the door latch or the factory rubber plugs that cover the access holes for mirrors, etc., and in the tailgate latch on both sides.
Just for laughs, if you have never washed the mud out of your truck like this before, do it sometime and watch the crap that comes out of the fenders, boxsides, doors, etc.. Here in Atlantic Canada, they use tons of salt on the roads, and all this and/or the mud that accumulates just rots away at the metal. Even with the boxed frames now it lays inside frames and eats away, and we all know that the metal used today isn't what was used years ago. It is thinner now than ever, and in my opinion, not near the quality, and doesn't last long without proper care and maintenance. In this part of the continent, and anywhere with the same conditions we have, it is not uncommon at all to see a three or four year old vehicle - any make or model with rust holes starting over the front or rear wheels, bottom of doors, etc..
Any rust prevention application, such as Rust Check or Krown, will help your vehicle last a lot longer than without. Unless you are lucky enough to live a dry warm climate, or have a garage with a hoist that you can put your vehicle in the air and hose it out underneath quite frequently. On reply mentioned that some brands make a mess and drips for a long time. That is because usually they use two different products for their system: a heavier one for the underbody, frame, any exposed area prone to wash off from water on the roads, and u thin runny product for inside panels so it can "creep" into places that the thicker products can't get to.
So IMO, anything is absolutely better than nothing, and my only warning to anybody that is going to get it done - YOUR MECHANIC IS PROBABLY NOT GOING TO LIKE YOU ANYMORE-----THEY DON'T LIKE IT BECAUSE IT IS VERY MESSY.
