The Unity uGUI system is quite powerful and once I get the hang of it, I has able to make the UI for an Inventory, a Shop, Dialogues, and other things for the player to interact with. But I also found it quite messy, with a lot of strange bugs happening.
So when I heard that Unity make a new system based on xml and css, I was really happy to found back something that I know really well.
After playing around for a few days, my opinion is a little more mixed. Mainly because you can’t place something “freely” on the screen like the uGUI. Since it is purely based on xml with a flex display (boxes inside others boxes), making something like a Talent Tree, or a Map was impossible.
For elements that can be done in a more … rectangular way, like menus, then I think it is possible to do a ingame UI with it.
But where I think it will shine is with editor windows. It will be much more easy to manage the display and the alignement of the elements that with having to write everytime and everywhere BeginVertical/BeginVertical, and having to code the GUIStyles.