I tried SuperMemo 2000 and found the GUI to be really, I mean really, difficult to use. Is there another interface that has been developed since then?
No. Currently SuperMemo continues with solutions inherited from SuperMemo 98. To use SuperMemo, you do not need to understand its interface, menus nor advanced functions. You can start off with just two keystrokes. Recently, two new interface solutions have been proposed to tackle complexity but none has been demonstrated to make life easier for beginners.