Works great and easy to use. Don't even have to think about it once it's set up.
I have this set up on a shop that fulfills physical items, but use this feature to make free PDF downloads available while still gathering emails.
Since I use it to provide free downloads to my customers, my only wish is that for free orders all they would have to provide is name and email address - rather than still going through checkout like regular orders. I think that turns some people off from completing the process.