They don't seem to be immensely popular with the people on this forum. Most seem to view these as the modern day "snake oil" - not worth it. You could check the archives, this was discussed before a few months ago, I think.
I used to work for a major west coast marketing firm. Companies can make money back guarantees on anything because they reserve the right to refuse any guarantee on the grounds of some retarded technicality or they add in the fine print stipulations that end up costing you more money to return it. ( Crazy shipping fees, Restocking fees ect.)