I looked into getting a Nectar card, saw that it only saves about 1%, and decided it wasn't worth the hassle of scanning. But then I did an actual calculation, assuming that it adds ten seconds to a shopping trip, and decided that it's an effective use of those ten seconds if I'm spending £3 or more. Now I have a Nectar card.

I didn't take into account the hassle of getting a card (a few minutes so far, I'm not sure whether I need to register it online at some point or the leaflet is just creatively written, so maybe longer), or of redeeming points (unknown) or of having an extra card in my wallet (small).