I replaced the rear pads on my MKS after about 5 years and 50,000 of service. They were BAD. Not only were the pads nearly worn to the metal, but the caliper pins were dry so it would not have been long until they wouldn't move. One of the inner pads was frozen to the caliper bracket. It had the most lining left on it since it really hadn't been touching the rotor for some time!
I had the Brake TSB done (front rotors, pads & caliper brackets) about 25,000 miles ago so the front brakes still have some service left in them. Granted, the original equipment brakes are not great but I really don't care to do a major upgrade at this point. I replaced the rear pads with Duralast Gold ceramic pads from Autozone. I put the same type of pad on my wife's Taurus last year and they are quiet, low dust and reasonably priced. Besides, Autozone loans the caliper piston tool that is a requirement for rear brakes.
If you have over 50,000 on your MKS or SHO and live in a climate where salt is used on the roads, you might want to check the rear brakes.