Screencasts can definitely be useful! Take the Compass screencast[1] for instance. When I first watched it I had no intention of using SaSS/Compass right then; it took me a few more months to find an opportunity to make use of it. What I did learn were the basics from a user's perspective. Sure, I could have read the API docs and browsed some example files, but why would I go to all that effort if I just wanted a gentle introduction to what it could do for me?

I've tried watching other screencasts which try to actually teach you how to do something and I have a tendency to forget them almost immediately. Take a subject like Vim: it's much more efficient to provide tips in text if you actually want to teach people how to use them. I'm not going to pause a video and try to make muscle memory for commands right then, but I might remember a tip on how to do X and reference a blog post later.

It's the difference between an "intro to X" and "how to do Y in X". Screencasts are great for gentle introductions; they're very difficult to make for anything more granular.

[1] http://wiki.github.com/chriseppstein/compass