I'd further add that in the time since I wrote that, where XUL has crept forward, GTK and especially QT have blazed forward. XUL just isn't remotely competitive unless your app looks like a web app, in which case, it probably should be a web app. The range where you need something that looks like a web app but isn't one, and absolutely positively won't ever need any of the extra functionality the other toolkits offer (all of which is there for a reason) is a pretty narrow one.
Between bindings and scriptability built into some of the toolkits such as QT, there's almost nothing left unique to the XUL platform. You can see an example of that in http://labs.trolltech.com/blogs/2008/12/02/widgets-enter-the... around 2:40, where the script driving the little Nazi is actually the scripting language QT ships with based on Ecmascript.
Between bindings and scriptability built into some of the toolkits such as QT, there's almost nothing left unique to the XUL platform. You can see an example of that in http://labs.trolltech.com/blogs/2008/12/02/widgets-enter-the... around 2:40, where the script driving the little Nazi is actually the scripting language QT ships with based on Ecmascript.