Not seen one myself (at an assembly/KH), but maybe that's because I haven't attended since they were introduced (or maybe they haven't reached England yet...anyone know?). Frankly I think it's crass. It makes the WT look more like the business (not religion) it is. Gone is the widow's mite. As other posters have pointed out, it just encourages people to donate more (I think we've all paid for things on a card which we wouldn't pay for in cash, as psychologically, we're not parting with money).
But then, I think THIS is crass too (begging for money from 'worldlies')...