I've started using stackz for non-Japanese things. I tried Supermemo for a while, but it is too complex without being smoothly implemented for my tastes. Although, as a strength, Supermemo can handle images, sounds, etc. If Stackz could also handle images (sounds are used much less frequently), it might make it more attractive for general learning, too. But that also gets much more complicated for use on a mobile device. Supermemo mobile versions don't handle images, so you can't use them on the go. I see Stackz as the best out there at what it does, which is quiz users on text. Plus it handles Japanese the best of any flashcard program.