Are they used at all?
As far as I can read on wiki, they are popular on Japan (on billboards).
I really like an idea like this:
1. you're walking on the street and see nice ad,
2. you take a photo of that and start reader app,
3. whoosh... you're on product's website.
It is definitely better than trying to type URL.
On the other hand: QR Codes might provide other usefull information (take a look on sample ZXing frontend