...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!
Also, I can easily perform text search for something in written docs, but not in videos.
Bottom line: written documentation is a must have, video documentation is very nice to have, but IMHO should never be a replacement, but rather an added bonus.
Of course, you may not have the time/resources to do all that so you will have to decide whats most valuable for your customers. Ideally, I'd say written docs hopefully with screenshots and at least one video, if you can.
However, what about tutorials for setting an app? Wouldn't you rather watch a setup video than read through a pdf?