With a little work I got a small sample powered by three.js running. I just lost a good chunk of time figuring out how to properly embed the WebGL canvas. I believe I’ll never comprehend HTML/CSS.
This is the gist of the code. Pretty nice, familiar looking Java code. And most importantly, written with all the powerful tools available in IntelliJ. I can’t live without auto-completion, live code analysis and automatic import-management anymore.
You “loose” the ability to share Java desktop and HTML client code, like its possible with frameworks as libGDX+GWT, but you gain access to many well known JS libraries. Debugging isn’t an experience as smooth as I’m accustomed to - yet.
Compile (transpile) times seem very good. With some sample code as small as this, most time is taken by the Gradle process.
The infrastructure (build plugins, candies) is pretty impressive already. Discovery of libraries isn’t great, as there are simply too many of them already, and there are no desriptions or filters for candies, or the DefinitelyTyped repository.
tl&dr: JSweet is worth keeping an eye on.