Earlier quoted context omitted.
IntelliJ uses a totally custom widget toolkit that actually looks great. But yeah, NetBeans isn't especially easy on the eyes.
I'm using Netbeans at work now, on a Mac, and the experience has been considerably less than ideal. The UI is almost, but not quite Mac-like (to be fair, when I've used it on Windows, the UI was almost-but-not-quite Windows-like) and I keep running up against things that seem like they should be configurable but are not. It's like Netbeans was designed by people who had UIs described to them, then semi-randomly decid…
Look at Sublime and Atom. Completely customizable and identical across platforms. I'd prefer that level of honesty to the fake-Mac interfaces that try and fail to look native.