My Take
For whole class lessons, at the upper elementary level, these videos are not as engaging for my students. They are not as entertaining as math videos from other sources, so I use them, but not on an everyday basis. I would not promote them as the primary teaching tool at the elementary level, but do still believe them to be a valuable resource for differentiation, review and additional support.
How I Use It
I use Khan Academy videos as a tool for review with concepts that need to be revisited or with students who need more support on a concept. The videos are clear and easy to understand. I've also used these videos as a resource for parents, who sometimes describe themselves as "rusty" with certain math concepts. They appreciate the videos as a review, so then they are able to help their children more easily.