It doesn't make as much difference as you'd think - at least, not on an everyday basis. I've had a reward card for a while, and on average it only takes two or three quid off a forty pound shop. You CAN get big deductions, but it depends on what you buy. They have card offers on wine and spirits, for example. But if you follow the yellow tags, and buy something that you normally wouldn't, well, it's not saving you money any longer, is it?
All a bit of a ploy, if you ask me. If they're losing customers to the likes of Lidl or Aldi, then perhaps they should try cutting their prices, rather than come up with some meaningless tracking system.