I'm mainly a Flex/Actionscript developer, I've also been learning Java but, I 'm still have an "beginner / intermediate" level in the way that I've only used Java for "client side" programming, mainly using Swing / SWT, or other stuff like processing...
Recently, I had a look at Groovy (after having used Ruby on Rails for a year) and I really like it. I was wondering about the learning curve for someone comming from the "Flex" world to "JavaFx" because for me (I installed the JavaFx SDK and started digging in) the "concepts" looked pretty familiar
I would say if you have a Flex background, you learning curve should be rather quick.
If you have Swing in your background, you may have to un-learn a few things, but once
you get into it you will not want to go back.