I can't believe how much math my kids have learned from KA. I tell people all the time that it's like having the best math teacher in the world at your desktop, all the time.
I remember one morning my youngest son rushed into our bedroom when we were still sleeping. "Daddy! Daddy! Wake up!" I jumped out of the bed: "What? Fire? Someone hurt?" He said "No! I just got an Earth badge on KA!"
My kids are far above their grade level and they have a true love for Math. I really believe none of this would have happened without Khan Academy. They've learned more than me or anyone else could have taught them -- and they've done it mostly on their own.
I love that kids get points for effort (listening to videos). Sometimes the main reason they listen to a video is because they get points. I also like that kids get points even if they don't get all of the problems right. I've noticed that if the penalty for getting something wrong is too severe, kids will just not even want to try. The points help console them when they occasionally "level down"
I even like that Sal made a few mistakes and the video points them out. My kids think Khan is a genius so seeing that geniuses make mistakes is good to promote a growth mindset. The other day I was mumbling to myself that I made a mistake and one of my kids said "It's ok Daddy -- even Khan makes mistakes"
That said I have a couple suggestions: my kids are most motivated by getting new avatars and badges. So they lost a little motivation when they unlocked all of the avatars (suggestion: have more avatars or something else they can do with their earned points).
I also think my kids will lose motivation when Missions are removed. I like the Course Mastery philosophy, but the UI is just not intuitive. In Course Mastery, my kids can't even find how out how many skills they are away from their next badges, which is the thing that motivates them most.