With some custom code and the help of libraries like MacWidgets, you can make some decent native-looking GUI's with Swing. See my profile for an example of one - it's a large-scale desktop application.
I concede though that if you go with out of the box LNF's, even Nimbus, your results will not be up to par.
I concede though that if you go with out of the box LNF's, even Nimbus, your results will not be up to par.