Excellent plugin, very simple to configure and works great. I used this to insert the Subscribe2 form into a pop-up using its shortcode [subscribe2], and it functions smoothly. Can add HTML to style text, limit the pop-up from showing up within a set time limit etc.
For mobile devices, just adjust the width to about 40% so that the 'X' will show up on any size screen, allowing the pop-up to be closed easily.
The only annoyance of the plugin is the in-your-face promotion of the premium version, otherwise it's a nifty tool.