QR codes have been around for a number of years. I wrote about using QR codes in the recruitment process
a while back and have used them for a number of client campaigns - but even with the best execution, the %CTR is very low.
One of the best things about QR codes is that you can embed nearly any type of content into the design. But there is a downside to this. You can maliciously disrupt others recruitment campaigns, download malware onto the users device, or send them to other malicious websites/applications!
This threat has been labeled "attaging" or "attack tagging". You might be interested to read a while paper on the subject called Using QR Tags to Attack Smart Phones