We currently use QR codes with some of our print promotional materials, but nothing that equates to any hugely significant amount of traffic. QR codes don't really have a tendency to go viral in any meaningful way.
Haven't used yet, but I guess they're a good idea for optimizing your print advertising space. The success also depends on the appeal of the corresponding website page. Remember people who scan QR codes are somehow in a rushy condition, so the landing page should have good calls-to-action.
I have a few tips on generating and using QR Codes:
1. If you are only going to generate one QR code, make the code using the Text option; the others are less flexible.
2. DON'T use the VCard option; really looks bad.
3. Post it on your web site as well as putting in on all your marketing materials, including packaging.