Given your experience I would not expect you to have a very high learning curve at all. Especially if you have ever used other rich experience tools such as Flex or Silverlight (I know they are vastely differently tools but the language styles are similar).
Your second question is a little harder to answer. On the surface, yes it is a client experience that is at the core of the tool. However JavaFX is rich enough, and incorporates enough Java that, these client apps can be very robust.