Actually the one thing that has put me off using one of these applications is the lack of availability of a library of decks and is probably the main advantage

Actually the one thing that has put me off using one of these applications is the lack of availability of a library of decks and is probably the main advantage of a web-based approach. I'm not a fan of the over-proliferation of social network-based sites (Halo's law: "Every site expands until it becomes a social network, those that don't are replaced by those that do"), but I think this is the key case where it could really work to both your and your users advantage. The hardcore users will be more likely to already have bought Supermemo, have lots of Supermemo decks and be unlikely to switch over. I think you could end up with a larger, broader market than that. But I think the concept is good.