Some things to consider or be aware of:

(1) Several of these home warranty companies may be owned by the same firm.

(2) An article in Realty Times (http://realtytimes.com/rtcpages/20030115_haddreport.htm) suggested  that the last warranty on this list has had some issues in the past. (2-10 HBW)

(3) Two homeowner chat groups are at www.hadd.com and www.hobb.org.

(4) Intero Real Estate Services recommends First American Home Warranty and Fidelity National Home Warranty Company.  

(5) I myself have had a very good experience with Old Republic Home Protection and my recommendation goes to them.

(6) Ask your friends, neighbors, and folks you know who've recently bought or sold homes about their experience with their home warranty company. Eighty percent of homes sold in California do include a home protection plan, so they may well have some input for you which would be valuable.

What Do They Cost?
It depends. There are two main types of coverage, Sellers and Buyers.

Sellers Coverage:  Home warranties can be purchased for the escrow period (when the home is "listed" or "under contract"). This is called  Seller's Coverage (the seller still owns the home). The cost is usually minimal - pennies a day - but the coverage is normally not as extensive as buyer's coverage (for after escrow is closed). But it can help a great deal if you have it in place when the inspections are done and the seller is surprised to learn about problems with the furnace, plumbing, etc.  Here's an example from my own experience. During inspections, let's say it's discovered that the furnace has a cracked heat exchanger. This is a bad surprise for the seller, as it costs about $1500 to replace the furnace, and it must be done (as a cracked heat exchanger can cause carbon monoxide poisoning).   During the seller's coverage period (once the home is listed and the agent or seller has ordered the coverage - whether it's pre-sale inspections or the inspections done once the home is in contract or is sales pending), many home warranties will cover $500 of this cost. The cost to purchase the seller's coverage is probably about $65 for a typical escrow period. So it provides good peace of mind for the "what ifs" of the inspections. It may not cover everything, but the seller will have saved money by having the warranty if something like this is discovered.

Buyers Coverage: The cost to provide a Buyer's Warranty runs from $300 - $500, depending on the type and size of home (condo vs single family dwelling, the amount of square footage) and the amenities covered (it usually costs more to include air conditioning, a hot tub, etc.). It lasts for a year, from the time the home warranty company is paid (or close of escrow, check each company to verify).  There are many possible "add ons" - some of which might be a very good idea for you to consider purchasing.  If that cracked heat exchanger discovery is made during the buyer's coverage, the home warranty company may cover all of it.