When the app first came out, one of the key features was its ability to sort a shopping list into aisle order for a particular supermarket, which was a major convenience.
Lists are built by adding items either through a nested category system (eg, Hot & Cold Beverages/Long Life juice/Orange), or by free text search (eg, entering 'chux' returns a list of Chux products. There seems to be a significant amount of data traffic during this process, so you'd probably want to build your shopping list via a Wi-Fi connection rather than relying on wireless data (eg, while you're on the train).
When you look up a particular item, the app tells you if it is not available from the selected store. What it could usefully do (but doesn't) is tell you the nearest stores that do stock it.
Items can also be selected by scanning barcodes (eg, when you're getting near the bottom of a packet of corn flakes), but that was a bit hit and miss in my testing. I'm inclined to blame the smartphone's camera rather than the app, as it seemed to work more reliably under fluorescent light than daylight.
Page 2: Some frustrations