It seems to me the UML module of NetBeans is a bit too much hidden. In NetBeans 6.5 it was very easy to create an UML diagram. No plugin installation necessary or sth. like.
Re
Visual Paradigm has released a community edition for UML 7.2 which is free for non-commercial use.
You can find it at http://www.visual-paradigm.com/download/vpuml.jsp?edition=ce. You have to register for an activation key though!