After fighting for a while with JavaFX, I've come to the conclusion that it isn't quite ready for me quite yet. Why? I've grown lazy.
The IDE tooling around JavaFX is still very early right now. Heck, even the documentation is still a little lacking around the reference side of things. For instance, that F3Applet class. I couldn't find any documentation on that thing anywhere. Plus, I ended up having to JAD the class just to understand how to configure it.
But back to the IDE. I know that content assist should be enough for anyone, but I'm more of the opinion that designing UI should be visual. Visual design cuts down on the time it takes see what a UI is going to look like. You don't have code something, and then compile/run it. You can immediately see what it is going to look like. Sure, you can use JavaFX Pad for some of this, but there isn't any palette of components to draw from in that tool. I have to just "know" what all the components are, and pull them in via typing. Ugh.
Adobe Flex seems to have this down quite well. It should, since it is a much more mature product compared to JavaFX. Sun will certainly get there with the tools, but they will be fighting an uphill battle against a technology that is moving with greater speed into the enterprise.