I love a loyalty card, I have loads lying around the place but I refuse to get a Sparks card as I have never heard anything positive about them at all.
I prefer a nice simple system - when you buy stuff we give you X points per £1 ... whenever you come in and shop you can either redeem those points at Y "point redemption rate" or just add more points to your balance. Every so often there can be offers of double/triple points either on the entire stock for a specific time or on specific items.
Those are the kind of loyalty systems I use - simple, straightforward, easy and I can use them even if I've nipped in on impulse on the way home.