I have had American Home Shield for around 9 years now. They replaced an air handler when it corroded out. I really need to replace the compressor too, as it is old and not strong or efficient enough to cool upstairs sufficiently. However, the air handler had to "match" the compressor so the whole system is insufficient. If the compressor goes out, I would probably have to get one that "matches" the insufficient air handler and so on. The proper (expensive) thing to do would be to replace both units simultaneously, but that 100% is not happening under home warranty.
I also had a water heater fail, causing water damage. The warranty covered the water heater itself, but no damage ancillary to the failure. At this time, I opted to replace the tank water heater with an on-demand water heater, which 1) cost more than the tank water heater, and 2) required an upgrade to my electrical box to support the higher power draw. The warranty company gave me a skimpy credit to apply toward my own handling of this installation.
These companies make their money by paying low prices for hardware, and replacing only what is absolutely required, and explicitly covered in their policies. They also are able to pay their contractors less, presumably with the offsetting benefit of providing steady work for them. My guess is that a contractor connected and talented enough to stay busy has no need of an affiliation with a warranty company.
Once in a while they will send somebody out who really doesn't feel like making an effort. When an issue becomes complex, they just want to get out of there instead of seeing it through to resolution. Other times, the person they send really takes ownership of the issue and has pride in their work. It'***** or miss.
All that said, I feel like I have gotten decent value from the service, and will continue to use them.
I like having someone to call. If I had any inclination to fixing crap around the house, I would do it myself. Spending $ on quality doesn't faze me, but I want someone who knows what they are doing.