I also quite like Sparks, but it seems to be a bit of a MN 'thing' for people to regularly post how shit it is.
Maybe people aren't shopping there often enough? My least favourite loyalty scheme, and supermarket for that matter, is Tesco. They tend to send me all my Clubcard points at once just before Christmas each year and tell me that I can 'splash out for Christmas' with the £1.50 that they've sent me. Or I might be able to save up about 3 years worth of points to get a half price Pizza Express pizza.