His videos helped me get through pre-calculus in college last year. Pretty cool stuff. However, when I asked my professor what she thought of the Khan Academy videos, she said he made a lot of mistakes, and if you aren't careful you'll wind up making those mistakes as well.
I found it useful just being able to rewind and play over confusing math topics, hearing him explain the same thing over and over, sort of drilling
the stuff into my head. That would be difficult in a classroom setting, when everyone else is ready to move on.
All in all, I recommend his vids on trigonometry to anyone struggling with the subject. Just be sure to check everything yourself. We all make