Excellent problems and practice, less useful for the videos themselves.

My Take

The problems and practice section of Khan Academy (KA) are superb. I've watched students get deeply engaged in this work and be very productive in "teaching themselves" via trying out problems, taking hints, and learning from their mistakes and successes. The fact that it is free, so comprehensive, and has great engineering and design behind the scenes helps a lot. The "lectures" or videos that Sal produces are less "sticky" for students. They tend not to want to spend class time watching the videos. An accidental lesson learned from Khan Academy may be that students need challenges and problems, a few resources and the ability to collaborate more than they need "explanations" of how to do math.