Had one at my first house (came with it), as others have said they aren't worth much and for the most part don't cover anything big. One example I was given was if there's a plumbing leak, the home warranty will pay to fix the pipe but not the water damage. What they do cover, they still charge a fee for a service call and send their choice of service/repair technician whereas I would prefer to use my own people or someone recommended by someone I trust, not just whoever has the contract with the warranty company.
As a general rule, I only insure things where there would be catastrophic financial loss in the event something happened (ie homeowners insurance for major fire or hurricane damage or a lawsuit) and I generally carry high deductibles. Insurance for small stuff, like a broken dishwasher, isn't worth it IMO unless it's free or nearly free. Insurance companies make money, they charge premiums greater than the expected payout, I strongly prefer to carry a large reserve fund and cover small-medium repairs out of pocket.