I find a written document, perhaps augmented with screenshots, to be most helpful. Screencasts, by themselves, I find to be next to useless. A screencast demands investment of significant time before one can even determine whether its going to be helpful. With a written document, it's usually possible to tell from the first paragraph or a quick skim if it's something worth spending more time on.

Screencasts are undoubtedly faster and easier to prepare than a written document, perhaps that's why I tend to see so many involving technologies such as CMS platforms that cater to folks who want quick solutions that they don't need to understand very deeply.