I'd like to add a very personal comment to this discussion. I have very bad vision. So much so that in school, I couldn't see the board even when sitting as close as possible. As a result, I'd end up creating notes from a synthesis of what the teacher was saying and what I read in the book as the lesson was going on. This usually didn't work out, and my math skills suffered.<p>This website helped me fill in the gaps of knowledge I had. Actually being able to see the problem being done while given instructions was a great help.