This used to be an excellent plugin.
Unfortunately I've recently had a load of complaints that this plugin does not function properly when using an iPhone or iPad to browse the site.
Firstly, I have set the option not to display on mobile browsers, but it still displays.
Secondly, the modal popup does not keep its position when scrolling, and cannot be closed. Basically, a big mess.
I've had to disable the plugin, which has been working fine for the last year, and I'll be looking for a replacement unless this is addressed.