This is my objective opinion as a contractor. I have no experience from the contractee side. I have dealt with many different home warranty companies, and one seems to stand above the rest for me.  First American Home Buyers Protection is my favorite because they seem to be the most fair for both the customer and the contractor.  I have been doing more service calls for them than all of the others combined.  You may ask why I can have an objective opinion since I work for them.  Fair enough question and I am prepared to answer that. I have consistently tried to put myself in peoples positions when it comes to service companies.
This is, of course, because I am one but I want to always evaluate the value to the customer and not just to the contractor.  I have been on many calls that only took two different sevice calls in one year and the whole year's premium would be used up.  A home warranty generally protects your cost by the fact that you only have to pay a small service fee per call and the home warranty covers the rest.  Often, the service fee is only about half of the smallest service call. 
When choosing a home warranty, always consider their claim coverages by checking with someone who has the same company and call them and ask them their experience with the company and how well do they pay claims. From my side of things, as the contractor, "First American" pays in a very timely fashion. They also rarely turn down paying for repairs.  I worked for the some of the other home warranty companies, and they seem to have tried everything to deny a claim.  The fact is sometimes with certain home warranty companies, I wonder what they will cover. Home warranty companies are essentionally insurance companies and the same thing applies.  It's not just how cheap is the premium, but also how well do they pay their claims.  This is why I say that First American is one of the best home warranty companies and my favorite one to work with. The only thing that really bites about home warranty companies is:  They are the ones to make the decision as to whether to repair or replace. Most often it is always to repair and repair and repair.  Also though, when it comes to replace, you are at their mercy as to what type of component they will replace with.  Most often they use the least expensive item they can buy. There is usually one option given you if you do not want to have the replacement item be chosen for you. Most companies offer what is called a buy out. This is the cost of the replacement item including the labor will be paid to you in a check and you can contract anyone you would like to put in whatever component you would like. Of course, you will likely be adding more than twice the buyout amount to put in a quality item including the labor.  But one must remember, any amount that you get is more than what you would have to take off of the bill had you had no home warranty company. Often, I find that it is a very good deal to have a home warranty especially if it is a good company.