-stops or covers rust on bare metal parts
-adds some sound deading overall
-some undercoating is needed in rear wheelwells of the '05's that drive on gravel roads alot (for noise)
-if you live and drive in the snow belt (and streets are salted) may be worth the extra protection
cons:
-most are applied very slopily and take away from the clean factory look
-overspray is a given even when carefull
-some places do the engine compartment (this looks like $hit don't do it)
-if coating is yellowish type avoid it, black/dark only
-adds a messurable amount of weight to car (extra weight = slower)
-any bolt on part on car that is covered is a pain to remove or work with
-even if they throw it in when buying, you are paying one way or another
-Ford has a very good rust through warrenty and if you ask a Ford engineer they would say its not nessesary
IF you have it done, get the car up on hoist go under it with the guy and show him the areas that you want sprayed, take wheels off and spray the wheel wells in the rear, don't spray any plastic parts like gas tank or shields etc. do spray the rear axle. make sure they have some sort of downdraft system to keep overspray off of car and they should cover and tape off completley. you may be suprised how much of undercarige is coated or painted already, all that can rust under normal conditions is axle and some suspension parts
No. The concept sounds great on paper, but it's just a high-profit margin item for your dealer with little or no proven benefit. If you're really worried, you could by a can of rustoleum and paint the underbody yourself.
If you're not driving in the winter, just get the drip-free undercoating. Also, make sure they don't drill holes in your car when they apply it. This is the advice I received.