Hi Todd,
Thanks for the link to iPopBox.
I have not used this software before and am not aware of anyone using it. To comment as to whether it is effective or not would be premature. Like any new software that is released, you need to test it. So I recommend you test it to see what results you get.
Ideally you would want to do a split test between the iPopBox and Hover Ad. Run both of them at the same time and see what opt in rates you get between them both. That's really the only way to see which is superior.
Again, squeeze pages are mainly used to build lists quickly. If your main goal is to build a list, then squeeze pages can be quite effective. Again, run a split test between a squeeze page and a hover ad on your main salescopy and then compare conversion results.
Test, test,test, is really the answer to all internet marketing strategies

All my best,
Geoff