I've tried quite a few of these Lightbox pop-ups, specifically to include a Mailchimp sign-up form. Because I'm using a responsive design it had to work in accordance with a twentythirteen child theme. I've tried quite a few over the last week and this one comes closest to anything half decent that can handle the mailchimp form.
My only gripe is the formatting of text within my form. When I add a paragraph the form stretches off the page. It would be nice to have an option to set the percentage width of the form based upon screen resolution.
Otherwise, a great free plugin.