I totally agree on the documentation / examples tip for jQuery and Mootools (but I'm still delving into those). I got a book on prototype and scriptaculous that really helped a lot, and there are some really cool tools being built by people using this framework such as LightView and ProtoTip, both by the same guy.
I really liked the look of mootools originally, but the documentation on that confused the hell out of me (I since have learned a lot about JSON and it makes things a lot more clear)...
Does anyone else feel like I do - I am pretty seriously dating Scriptaculous right now, and I don't know if I want to get married just yet, that is before I have a chance to take Mootools and JQuery out on a couple of really serious dates?