My Take
While I believe the lessons can be helpful, I've often found the lack of personalized instruction made it seem like I was just learning the same thing I was in class. I would often still feel stuck even with the option for reading text for extra help. However, I believe that if integrated well with a classroom, this means having assigned work or assigned extra help from a teacher on this resource, then it would make the best combination. Having a teacher as a guide that is working/integrating Khan Academy will help much more than just doing it on your own.
How I Use It
I would typically use Khan Academy for the math subject since it was often the subject that I needed the most help with. I have also attempted to use Khan Academy as a study guide and to try and get ahead in the topics that were going to be taught in my math class. However, I found that it didn't often work because of the fast pace of these lessons.