Right now it is Prawn[0]. It slowly evolved from a script a year ago, where it was fine. We are considering switching to an HTML->PDF workflow, though, since I have to render all of this stuff in the web for quicker previews anyway!
Thanks for the clean design mention, we owe it mostly to Zurb Foundation[1]. Using those modal popovers to control the user's attention has really helped me narrow the UI variables.