I've been playing with XUL for a couple of days now. Kind of interesting and kind of really confusing.
It looks like it could be really useful for making cross-platform applications and the way it works means that it's not a huge jump from HTML development to XUL layouts but getting plugins to work on OS X? Ugh.
The large number of folders called 'plugins' within the standard application structure definitely didn't help. At the moment, my application works but I have no idea why. It's probably a quantum thing, you can either know what it does or how it does it but not both.
Still, it's available for download on the (almost finished) MonkeyTV site. Now I just need to go through the process again for Windows...