Anyone got any recommendations out there as to the best method for
building a windows GUI in Java?

I've been looking at the XUL technology and it looks interesting. I'm
wondering if the technology if mature enough & if one can use it for
building a frontend/GUI (standalone) app. I'm not looking for something
which is going to require a lot of hacking, I'm just looking for a
fairly mature technology. (Anyone successfully used the Apache Jelly
technology?)

Otherwise the Eclipse SWT technology looks interesting and they have a
free plugin for UI-dev.