Of course, Khan went on to hire Hart, a partnership which could be yielding all kinds of fruit. If anybody has noticed Khan innovating on his format since he picked up Hart, drop me a line in the comments.
The KA lectures may or may not be effective in helping people with math, depending on your idea of “helping”. The KA problem sets are almost certainly going to give many students the impression that they know and understand something that they do not. Naive teachers may be taken for a walk along this primrose path as well. For that reason I think the problem sets could be quite destructive. I don’t object to checking for mastery, but KA doesn’t even come close to having assessments that measure mastery.