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.

Especially bad is when the presenter shows a desktop for the first 2 or 3 minutes while talking about what they are going to be talking about.