Probably not much unless you're upgrading your design apps. Every newspaper that I send ads to still specifies V4 compatablitiy. Every printer that I've ever sent a pdf to has specified 4 or 5 - never 6. I went from 4 to 6 (came with Adobe creative suite). V6 has very extensive preflighting and the ability to preview separations, Don't know if 5 had that.
I think most of the additions (deitable forms, intracorporate type stuff, etc) to Acrobat have more to do with things other than just outputting for print.
If you're upgrading all your design apps (and considering Indesign) and you use Adobe, their creative suite pro $750 upgrade from any version of Photoshop is a pretty good deal.