
Java Swing comes with "pluggable look-and-feel technology", which essentially boils down to the fact that interfaces can be "skinned" (although this is simplifying a tad) and is therefore, extremely flexible. By default, Java ships with a cross-platform look-and-feel (LAF), which means your apps can look consistent across all platforms, or LAFs that mimic the look of a specific platform, say Windows, for example. However, one of the chief complaints of Java desktop applications is its "look". It basically stems from two issues:
Either your machine is extremely slow or you're running an older JVM, but I use Swing apps all the time and honestly I don't notice a difference.
Netbeans
RSSOwl (SWT though)
JAlbum
LimeWire
To be honest the speed isn't that bad