I've just signed up for a membership and have downloaded some plugins and themes which all seem pretty awesome, really powerful, and seriously lacking in documentation.
It's not a swipe at WPMUdev as it seems that all WordPress plugins and themes don't come with instructions of any real usefulness. I'm not sure why it is. Lack of time perhaps?
In my daytime job I need to work out how audio visual equipment works and am constantly referring to equipment manuals all day long. I couldn't do my job without them. I seriously can't see how plugins and themes are any different. If there were in depth manuals on how these things worked it would make everybody's life a lot easier. You guys would spend less time answering questions and the people buying this stuff could figure it out themselves!
A manual that goes right into the nitty-gritty of functionality of the theme or plugin would be awesome, rather than the fairly basic manuals that only briefly explain how it should work.
Just my two cents.