Hello! My name is Humberto Valle, I am the founder of www.Unthink.me a strategy formulation and marketing agency. I think that the majority of our clients are companies that sell intangible goods such as software or services. One of the biggest assurances a company can give to prospect clients is their complete and overwhelming satisfaction.
In the years that I have been helping companies formulate strategies for growth competency I don't think I've seen one case that a consumer takes blunt advantage of a guarantee like this. The reality of things is that it will and at least could happen but not to where your business will really hurt from it. Specially if your customer service skills are above par and your work/product is outstanding. I say do it, think about it most companies that use this perfectly tend to offer crazy scary (to them) promotions.
We have a client that offers 200% money back. lol. Has never had to use it in their 3 years since implementing.