If you can, shop on their online shop, the differences in price for most things is ridiculous.
But don't call people morons who shop there, they might not like shopping online and using their card(and don't want to ask others to do it for them), or they like shopping in an actual shop... and believe it or not, there are people who still don't have the internet in their homes.
As for supermarkets, they don't have an awful lot of choice, and they usually(if you are like me) don't have the editions I would like(steelbook for example).