So, without further ado, the positives!
- The API is nice and clean - and not just because it's not stuffed with deprecated methods like Swing is, it's just designed in a fundamentally much more sensible way which makes it easier to follow. The concept of properties on elements, with these properties having a common interface, means I can jump right in with a component and see what properties are available to me without having to dig in the documentation to find out exactly how to add a listener for the width of the bottom half of my split pane (for instance.)
- Layout is vastly improved - a lot of the swing inconsistencies have gone, and the model is now a specific component is tied to a specific layout (HBox, VBox, GridPane and so on) rather than having a separate content pane which can have a layout applied to it. This again ties in with the first point, making for a much easier, nicer API without having to wonder exactly what, if any parameters I need to shove on the end to get the layout manager which I've selected (and what one was it, anyway?) to behave.
- You can actually do animations with keyframes without resorting to horrible graphics2d hacks - and it's all GPU accelerated.
- Multimedia support comes as standard, no need to play around with the buggy as hell Javasound or JMF.
- UI work on the platform thread is enforced most of the time - a runtime exception is thrown if you don't do this, which makes it much easier to find and solve odd annoying concurrency bugs that would often crop up otherwise.
- The default cross platform skin doesn't make me want to vomit. On the contrary, it actually does a good job of looking rather nice across a range of platforms. It's visually appealing and nicely animated too.
- Nice native deployment options - they only work on the current platform (i.e. you can't build a deb package on a windows box) but still, I like the change of thought that users are generally much more comfortable with a custom built package for their OS rather than a generic jar / jnlp file.
Despite the above, it's not flawless:
- The multimedia support is great - when it works. But it supports in reality a very limited range of formats and file types (no mkv at all for instance, some of my mp4 encoded videos still didn't work either.)
- There's still a few annoying bugs I've come across, such as this one. Nothing that can't be sorted, but these sorts of things are annoying.
- Some features just aren't there yet - I wanted a pop up panel similar to the ColorPane, but those components just don't exist yet (or at least aren't part of the public API.)
- No rich text on controls, at least not yet. This gets really annoying if you want to (for instance) bold part of a label. Can't be done, you have to butt multiple labels together in a HBox to get that effect (which, let's face it, is nasty.)
- No native skins - you can write your own, but at the moment (as far as I know) none are provided to make it look like the native platform you're working on. Some would argue this is a good thing, but sometimes it's nice to have this option.
- No damn font metrics in the public API - again, something I have to use an internal class for at present (which is really rather annoying and means code I write at present potentially isn't backwards compatible.)
Overall, I must admit I still like it, and while I've uncovered more things I don't like from my initial positive reaction, most of those things are slated for inclusion in Java 8 (so in a year or so at the time of writing.) I still don't think it's going to take off on the mobile or web front - but as a replacement for Swing on the desktop, it's a very welcome (and arguably long overdue) change.