I do it too but only on small things. I haven't done a single refund yet over several hundred sales.
Other than increasing the likelihood of a sale, the benefit of offering a guarantee like this is you get less flack from your payment provider (since they can see you'll refund if anyone's dissatisfied) and you can avoid nearly all chargebacks. If someone doesn't want to pay, they'll find a way to get around it anyway, so you might as well make it frictionless.