Ron,
QR Codes have been used in the automotive industry (I'm thinking Toyota, but I may be wrong) for close to 20 years now, but several years back they became popular with smartphone users because of their ability to be very quickly scanned.
I think they seemed to rise to the public consciousness because of the "easter egg" factor. Before they were wildly popular, someone could leave a secret message in plain site (invite to a party, opening to a new club, etc.) right out in front of everyone, and only those with the app to decode them could read the message. Think back to the decoder rings in the cereal box days. There was a definite cool factor.
While they are still used for that--sometimes stores (especially in certain neighborhoods) will offer very steep discounts, or a hidden message to their customers using them--marketers have now picked up on the cool factor and it's a great way to get a lot of information in a small amount of space.
Some of the things I use them for with my clients:
I have a client who does limousine service to the NJ and NY airports. He has one-page "business card" site (basically an one-page site containing basic information that a business card would) A small QR code on each of his vehicles allows his clients to scan it, be taken to a screen in their browser which prompts them to save his business card site to their phone. Many of his clients use him several times per month. Now they can tap the icon on their home screen, and be directed to his mobile page, which gives them phone numbers, etc.
I have a client who is a small Italian sandwich shop in NJ. A small QR code on his menus, mailers, and door directs his customers to his mobile site where they can view the menu, see his hours, call for delivery, etc. They also have the option to save a direct icon on their home screen to go there (handy for office managers near him who may be using him frequently).
One of my clients has a QR code on his business card. Business cards get lost, but the QR code has his basic contact information so that they can save his phone number in their phones and contact him when they need him.
I noticed on TV the other day, Android had a QR code that flashed for just one frame. When I paused and scanned it, it was for a scavenger hunt. You had to have an android phone to participate though, and I didn't but I did think it was brilliant.
I have a friend who uses them to mark sections of her flower gardens. She keeps notes in her phone, and each scan takes her directly to that plant's information page, etc.
My cable company puts them on my bill, and clicking on them tells me what's onDemand this month.
There are a lot of possibilities to engage your audience instead of them ignoring your ad. They want to read the secret message.