Personally I'm happy with how IntelliJ IDEA looks on my Mac or on my Ubuntu.

It can be sluggish but I've never met an IDE that isn't sluggish and that's worth the price of me not using a text editor such as Emacs.

This "Java UI" that they use give them a competitive advantage - it makes it easier for them to support multiple platforms. I'm happy that I get to use the same IDE on multiple OSes, as I routinely use multiple OSes (e.g. for work I now use OS X, for personal projects I'm using Ubuntu Linux and from time to time I use Windows too).

What other IDE has a comparable feature set, works on Linux, OS X and Windows and does not suck like Eclipse does?