The default theme on Ubuntu (I'm on Natty) is Ambiance, which means NetBeans IDE with Nimbus looks like this:
If you use Crux, Inverted, Simple, or Redmond the IDE looks better, e.g., you can see separators in the menus and the menus aren't all black:
At the moment, I like Crux best of all!
On the other hand, Clear Looks, New Wave and Radiance look less good, either because of missing separators or because of the dark menus:
Two other themes, "Dust" and "Dust Sand", don't show menu separators either.