Hey, guys, thanks for the feedback.
A plugin like this is hard to get working on all setups, which is why I tried to include many options for delivery and templating.
The newest version is a bit more stable. You can try that.
One feature of the "link by email" option is that the user needs to use the same browser they logged their email with to open the file link (since the plugin saves a session variable noting that that user is okay to get the file). This is kind of a lot to ask for some people. Their default browser may be different from the one they logged. They might get their email on a different computer. They might have closed their browser (and the session) so the link doesn't work.
I would guess this is why you were getting random results in your testing.
This delivery method could be improved a number of ways (I'm not planning on it though). I would suggest using a different delivery method.
Paul has a good point RE getting valid email addresses. Sending the file by email is the best way to assure there is a human on the other end of the email address (although throw away addresses could be used).
That won't work on some setups though, so sending the link or showing the link on the page will at least get you the email address of people or aren't trying to scam you for a free download. Depending on your subject matter, I would guess that most people are going to give you a legitimate email address that you can follow up on.