I think the idea of the database that knows what words you know is kind of a fascinating idea in a way, in that it goes beyond personal information into personal knowledge, a digital reflection of your actual understanding which corrects itself continuously to reflect it more accurately.

Anyway, if this was a service I could actually use online, I most definitely would, and I might even pay for it (as I once did with smart.fm which became paid for a year or two ago). I'm currently learning german quite intensively, and anything that makes this highly laborious process (that of cramming new knowledge into my mind and trying to make it stick) more efficient is extremely useful to any language learner.