I'm a high school math teacher.
Here's my experience: Like anything else, the Khan Academy videos, and everything else like them, are a tool. /their value lies in the application.
I have no plan to ever completely "flip" my classroom. But I did start this year with kind of a "slow rollover."
Two or three times a week (at least prior to Hurricane Sandy-- she's put a bit of a crimp into my planning) my homework would be for my kids to watch a Khan Academy video, or some other similar video. They were to take notes, and I checked those notes the next day.
The next day in class, I went over the notes, but did so kind of quickly. There were no huge pauses as I waited for my kids to copy all those geometric theorems. But I DID explain each and every one. The kids were already familiar with the explanations, having seen them the night before. But I went over each one again, to ensure that the kids didn't have any additional questions not covered in the video. I also supplemented the information presented. Like most teachers, my own explanations and hints vary from my peers. I like to include those hints and shortcuts in the notes.
But, since they had already taken the vast majority of the notes, we had a whole lot more time for practice problems.
I loved it, and the kids loved it. It was a nice change for them-- it kind of broke up the homework load. I plan to go back to it next year (when I'm more sure that all my kids have access to a computer. Right now I still have a number who are displaced, and possibly a few still living in a shelter. So for the time being, I'm sticking to the textbook for homework.)
Like any other tool, the videos can be misused. But I think they're a great addition to the tools at my disposal to help my kids learn math.