When I negotiate a warranty into a contract, my first preference is to use HMS. Past clients seem to have good things to say about their warranty. Customer service tends to be good.
I bought my home in 2004 and I've renewed my warranty every year. It's more than paid for itself. I did have one horrible experience with a certain contractor. HMS gave me a new contractor and waived that deductible.
Overall, I'd use them when I can...