Technofreak:True most tablets are OK on a full site but it's my experience that many phones while being able to display a full site certainly don't do it justice and it can be fiddly looking around a page and other than geeks like us the average person won't bother looking around a website that's not ideally suited to a mobile device.
The nature of a QR code is that it can be placed almost anywhere to provide a link to a site without having to type in a web address and avoid the high probability of mistyping the address. Therefore the most obvious user device is a mobile phone since tablets are not carried around all the time like a phone is.
My point being those using QR codes should ensure that (A) the QR is of appropriate size and location to make it easy to scan and (B) the website it goes to provides a good user experience, i.e. is is optimised or scalable, otherwise its a fail.
The problem with some mobile websites is that they can cut down on the information on the page, so you don't see it all. Also some responsive website, you may not then be able to switch to the full site, as unlike a website that has both a regular and mobile version of the website. resposive sites also scale the website down to different formats for different screen resolutions.