The thing that made my experience with mootools a rather unpleasant one was the documentation and the stability of the API:
I simply wasn't able to find a documentation that related to the mootools-Version in use. Won't be that much of a problem if the API defined was stable. But due to some functions that disappeared in the newer Version (a ChangeLog was found after hours of searching) a migration wasn't possible either. After that, mootools was out of the race for me.
Like many others, I don't want to introduce class-based OOP into simple user-interface manipulations. Thats what I use jQuery for: not so complicated user-interface stuff.
When I have to build rich browser-side applications, I'd always switch to the big solutions (ExtJS, YUI, qooxdoo) that offer a variety of ready to use widgets.
