In my quest to make the world a slightly better place, there is new wiki article: scene2d
This will get you acquainted with the scene2d basics. It doesn’t touch on the scene2d.ui package at all, since that is quite complex and needs its own wiki page.
Overall most people seemed generally open to improving scene2d. Hopefully this means you won’t be too upset when you update libgdx’s SVN and have errors in your project! The changes so far should not be too bad at all. Only the scene2d.ui package has been touched. It now has a proper API — all fields are private and are exposed as necessary. This unsurprisingly turned out very clean and means less breakage in the future, since implementations are not exposed. Some other minor changes were made to keep things organized and consistent. ComboBox was misnamed and so becomes SelectBox so we don’t have problems when we add an actual combo box later. scene2d.ui is fully javadoc’ed (except for the TextField, Widget, WidgetGroup, and Window (and Table) classes which will be done soon). These are javadocs so don’t expect to find glamorous tutorials, but it should prove helpful. We do realize that the scene2d.ui stuff is complex enough that a detailed wiki page is needed. That should be coming soon… or at least eventually.
A while ago I added a class called Scaling to the utils package. This handles up or down scaling a source rectange to a target rectangle. Turns out I accidentally had the functionality for Scaling.fit and Scaling.fill swapped, so if you are using this class and are wondering why all your stuff is scaled incorrectly, it’s because I fixed it.