Never never never buy anything but a factory backed one of whatever brand you choose. The aftermarket warranties are notorious for screwing people. I've heard of a few success stories of some through credit unions and Costco but it's just not worth the risk. These aftermarket companies are geniuses at hiding ways out of just about everything in the fine print. Also, they regularly just go out of business leaving you SOL.
As for whether or not to buy - of course, it's a losing business model for consumers. All insurance is but most of us can't absorb the huge hit of something catastrophic so we pool our money together (and give a bunch to the administrating company along the way).
While I generally run from most warranties I did buy one on my Ram and am glad I did. As people have mentioned, these trucks are just so high tech and expensive to work on that it's becoming pretty scary to run with one without any coverage.
One thing I can tell you from my own experience is I do get a lot of piece of mind by having the warranty. If I ever hear a funny noise or think something is going wrong I honestly don't care. My biggest fear is being stranded somewhere with my RV out of cell phone range. Given what these trucks cost, the % of an extended warranty is not that much.
