Used correctly, a QRC is one way to imbue offline, static communications with the brand-building power of sight, sound and motion — e.g., a link to an emotional brand video, or a provocative product demonstration, or compelling customer testimonials. (For the record, I'm no QR fanboy. I believe an SMS call to action is a more natural behavior for most people. Not everyone has a barcode reader, but all know how to text.)
Use it incorrectly and you end up doing something like Medica has done, which recently wallpapered Minneapolis with its QR codes.
After a couple of weeks of curiosity (one of the secret draws of a QR) I broke down and tagged the code. Imagine my joy when it revealed a mobile site that can give me a health insurance quote. I feel this misses a QR's true potential and also overlooks the role of context in brand-building — a parking garage usually is not the place where I want a price quote on my health plan. (Those more mundane moments in life may be the exact time to inspire me with tips and tricks to live healthier.)
If you want prospects to go to a microsite, simply give them a URL.