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.
Stick with Prawn, as it is sanity saving for this task. For web based previews, use GraphicsMagick and Postscript to turn the PDF into a gif or image. One command line, takes half a second. Saves you from doing CSS hell on layout, where the preview pixel wise matching the print data really matters for your app.
Thanks for the tip, Patrick! I had heard the html->pdf tools were pretty good, but you're absolutely right about the pixel precision needs. I got my Prawn setup into a reasonable state pretty early, and it's be Mostly Fine, Just Fine.