SWT is pretty good, and the fact that it uses native widgets makes it great for accessibility. Creating a well-behaved menu bar on OS X was tricky in my experience though. Also, how would you package the app? Ship the full Oracle JRE, or use something more lightweight like Avian?
I work as a web dev, I'm only a hobbyist wrt desktop programming, despite having worked on a small commercial app in Delphi. The toy apps I've built with SWT were tested only on Windows. I didn't even know about Avian - nice reference. But if I had to ship an app, i'd leave it up to the user to install JRE, on the installation instructions.
