I did it once for a conference in the UK. It linked to a google site I set up with PDFs of the poster and reference papers. However, I only really remember one person commenting on it and scanning it. Lots of the...lets say...older generation didnt seem to know what it was.
Also, at a conference in Spain, I noticed some on posters, but heard people commenting they didnt want to scan in case it might incur mobile network charges (myself included).
If you do use one, dont do what I did and put it on the bottom - place it on the top of your poster, so its more visible and easier to scan! :)