Well, I think allot of it depends on how you use the vehicle too.
I bought my '06 GMC Sierra 2500 HD new at the end of '07. The bumper to bumper warranty for GMC then sucked, I think it was 3 years and whatever mileage. The thing is, I only use it to tow the GN or if I need to move something. So, it only has 35K miles on it and I really wanted to draw out the years some more.
I priced GM and whatever it was the dealership was pushing and they were both around $2500 and sucked as far as coverage went.
Someone on the GMC forums turned me on to a warranty through Mercury Insurance Group. It only cost me $1500 for 5 additional years and up to 100K miles, which is perfect for me since I don't drive it regularly. And it's super user friendly for you and the shop. My steering gear box **** itself and I had to use the extended warranty to fix it. The GMC dealer called them, told them who they were, where they were located, and some info about my truck. They paid the bill over the phone right there and didn't even ask for them to send them the paperwork. The shop guy was dumbfounded...he said it was the easiest experience he'd had with an aftermarket carrier. He said GM probably would have sent out an adjuster to look it over before they paid.