Nexus 7 has only low-res front facing camera with 1.2 megapixel and a number of QR code apps I have tried use the rear camera and don't work with the tablet. That makes it hard to read barcodes. However, Barcode Scanner+ (Plus) app by Sean Owen (of ZXing Team), which is a learning curve to using a front camera, works well on Nexus 7.
You can also use it to find out product's prices and reviews. But I tried to scann linear bar codes on the back of books and the app did not work. There are some issues with the app, such as good light levels needs, difficult to focus, not read with curved surfaces code.