IMHO it was actually quite easy to use and had very sensible abstractions, however it failed to look good anywhere. SWT was much better in this regard (but much less fun to use).
It was straightforward to use. I wouldn't call it easy by any stretch of the imagination--you're still trapped into Java-style callbacks. It also has all the classic problems of event loop oriented frameworks, meaning you're manually managing massive state machines. IIRC there was no attempt to assist organizing the data flow across user actions except via traditional threading coordination mechanisms.
Well, I wouldn't use a closureless/continuationless language for a UI except as a platform for a higher language. This is 2016 and the lack of such primitives are absurd.
For the time, Swing was pretty good. Things have moved on and now data flow and state management are primary selling points—see things like CoreData (I'm not really aware of anything like it in other languages) or two-way binding that's prevalent in javascript libraries. This by no means solves state management but I think the primitives are easier to reason with.
