I've started looking at libgdx this month.
It piqued my interest mainly because it's supposed to have a built-in GUI library, something sorely lacking in many game libraries. Its documentation leaves something to be desired though. For example, setting up a UI apparently requires you to initialize a Skin, which is defined by a texture atlas and a JSON data file, but I haven't seen an explanation on how to write this data file. I get the impression that most users are copying the skin data file and texture atlas from one of libgdx's example projects and just tweak the atlas.
Edit: No wait, the Skin doc is right here. A bit sparse, but not completely hopeless.
libgdx also seems to have had an update to its API, so a few online tutorials are now out of date.