I taught gifted elementary students for over 25 years and feel my 4th-6th grade students benefited greatly from using Khan Academy with some direction from me.
The first time I used Khan Academy was with my sixth graders who were having trouble understanding why the distributive property was true. I showed them the Khan Academy's video on the topic so they could get a different perspective. The video really clicked with them.
Note that in general the videos explain concepts, but too often students just do the exercises and look at hints, resulting in much less conceptual understanding. I didn't have this problem with my students because Khan Academy supplemented what I taught and my students were often interested in the videos as a way to supplement understanding. In other classrooms, I witnessed students ignoring the videos going straight to the exercises and trying to learn from exercises and hints.
My gifted students were often perfectionists who couldn't also admit that they made mistakes. Careless mistakes often were dismissed by students who said, "I understand it, but I was just careless." Having to do 5 exercises in a row that were correct, with the computer grading them, was a way to insure that they really mastered accuracy and facts which are so important at the elementary and middle school level. Having to come back the next day to get mastery badges and points, motivated them to keep plugging away at skills they might not have practiced otherwise.
Gifted students who were ahead of the class were given chromebooks to explore new topics and practice old ones. This made running the class so much easier. I was also able to give everyone a chromebook and differentiate among the students. Everyone was doing the topics I recommended for them on Khan Academy. One advantage of Khan Academy for the teacher is that you can recommend topics to your students.
Students were also happy to get Khan Academy for homework. This is homework that I didn't have to grade, because the computer did it for me. I could check on the students and see how much time they spent at home on the topics that I assigned and how much trouble they had with the topic.
In summary, Khan Academy has wonderful tools for the teacher including recommending topics, tracking student time and progress, and built in grading. Elementary students can develop accuracy, become motivated, gain understanding, and build confidence.
I also enjoyed Khan Academy and spent countless hours exploring different topics.