I don't know about other makes, but I've never heard of a Mercedes dealer selling an aftermarket warranty. Mercedes offers a wide range of factory warranty products, and if a used car has an extended warranty, that warranty coverage is going to be excellent. I've owned a few brands, and I've never had warranty service like Mercedes. Dealers always go over the car with a fine toothed comb, looking for anything that might be covered. Also, Mercedes never took shortcuts when it came to repairs. I had a few instances where MB Canada could have saved bucks through a repair, but instead replaced very expensive components.
My Dad bought a CPO Mercedes, and it wasn't much more than he would have paid anywhere else for one with similar mileage/condition/records, etc.
We looked at some CPO Lexus RX models, and they all had very detailed/excellent service records.
I don't think the Subaru CPO program is worth a nickle. I looked at a late model Outback, and it seemed no better than any other used Outback, had limited records, and they were asking a huge premium.