That's a too general question i think. custom editors can be used for many different things. Basically it's like the runtime GUI system with some more cool controls. If you use C# intelliSense should guide you through most functions without the need to look into the documentation.
It completely depends on what you want to do. And there's no clear deffinition of "advanced" ;)
The community already shared a lot of quite useful and informative Editor Scripts.
answered Jun 21 '11 at 02:33 PM