This plug-in is well done and quite easy to setup in order to start selling anything that can be downloaded. I used it for music and translated it in French. It includes the Paypal sandbox for tests, and until now, everything worked fine.
On the graphical design side, the result will highly depend on your theme… for my site, I had to do some CSS modifications. But it is quite easy, and its “template” approach allows anybody with some HTML/CSS knowledge to create his own layout.

The security on the download link is low, but this is mainly due to the WordPress design which is not built on security requirements… the system of download link with email confirmation is quite limited in terms of security, but enough for most cases.