I think geography is important, awareness of QR codes is still not all that high amongst many people in the UK, and sometimes it's industry dependent, like tech and media/creative firms have been using them for ages but everyone else was much slower on the take up.
If you were doing a campaign in central London on the tube I think it's worth it; people are in a hurry so it's functionality is useful, huge numbers of a wide diversity of people including many "early-adopters" like media/tech/students use the tube etc. By comparison if you run it in rural Yorkshire you'll get a really poor response.
On the whole I think they need to be used a bit more creatively, there is a fad where people bung on a QR code that just links to their website, if that's all you get people will stop using QR codes. Meanwhile various bands have linked them into MP3 downloads and things like that, if QR campaigns consistently offered something interesting it can become an ongoing and very useful marketing technique.