I gave up on Nifty myself, now remodeling using Lemur. Currently working on a stock exchange interface concept:
Aside from minor hickups and containers not putting stuff where I want it to be, Lemur has been pretty awesome so far. A lot closer to the ideal JavaFX 8 gui than Nifty and even has event handlers for buttons.
My only critique would be the styling system which is way overly complicated for my taste, haven't managed to make anything remotely good looking with it yet. Thus I still use the glass theme like everyone else lol.