I have nothing to say on the merits of Khan Academy (I have never looked at it, nor have I known anyone taking a course), but from 35 years of teaching, I know

I have nothing to say on the merits of Khan Academy (I have never looked at it, nor have I known anyone taking a course), but from 35 years of teaching, I know that we mostly learn by doing. It's why university courses have labs, problem sets, and required essays. All else is memorization (not that memorization is inherently bad, part of learning assembly language programming is memorizing the available instructions, while learning French includes memorizing vocabulary). Educational technology that is focused on delivering “content” is unlikely to be of much value. Technology that induces the learner to experiment, to discover, to discern patterns, and to ask further questions, is going to be much more valuable.