When we purchased our home several years ago, our real estate agent bought us a home warranty as housewarming gift. The cost was somewhere around $300, if I recall correctly, and was from a company called Hisco. We actually used it on a handful of occasions, and the cost of the warranty was definitely covered by these service calls.
We renewed the warranty for the next couple years. Keep in mind that for some home warranties, the renewal coverage (i.e. policies beyond the first year of home ownership) is not as good as the new homeowner warranties. There's a lot more restrictions as to what is covered. We stopped renewing the policy when we realized that we weren't recovering the premium cost anymore, and when we got to the point that anything that went bad (furnace, washing machine, etc) would be replaced and not repaired.
The response time of the contractors through Hisco was actually very good. But I've heard that other companies are not as quick to respond. You're also restricted in your choice of service providers. Actually, you have no choice whatsoever - the warranty company will send whomever they've contracted with.
There's typically a copayment of sorts that goes along with a warranty service call - around $25-50, depending on the company.
IMO, if you're reasonably handy with minor repairs, then a warranty may not be necessary.
The are good because you just never know and for the most part the are fairly inexpensive ($299-$599) and cover a wide range of things in your house...nothing worse than moving in and a week later your hotwater heater/ac/fridge/etc goes out....ours was included in our closing cost that the sellers paid.