As has been said: Good written documentation is essential. If only for ease of looking at an index to jump straight to a specific section instead of having to 'guess' what part of a video I need to skip through to watch a specific bit...
...that said, screencasts and the like can be invaluable for actually demonstrating things like using convoluted software or coding for example!
Ideal world: Thorough, clear and well organised written documents are a must, use videos to 'highlight' things that don't come across well in the documentation or demo things!