thank you for creating this plugin. I do have one problem when using it in conjunction with w3 total cache, and I think it's due to your design decision to use php sessions instead of browser cookies for the "display only once" mechanism.
In short, it's not compatible with w3tc.
Situation 1: If I clear the cache and visit my site, the plugin shows (since its "display=YES" code is triggered. w3tc now saves this state into the cache, so on any subsequent visit, it will *always* show the pop-up (instead of only once), to every visitor.
If I now go to another page, the display=NO code is now in effect, and that other page will NOT show the pop-up (as expected). However, since now the "no-popup" version is saved into w3tc, any new/other visitor that enters via this page will *never* get to see the pop-up.