I've always had a good experience with Home Warranty of America -- the only problem I had was when they claimed a deductible was not paid and they discontinued the warranty.
Receipt was produced and they continued it but I was not too happy that they did not apologize for their mistake. Rule #1 - make sure you keep the receipts of the deductible you pay. (Not that anybody I know of is going to do any work before you pay the deductible anyways.)
Other then that, I've always been happy with their customer service.