I'm using BoUML for UML modeling on a current project.  I'd previously used ArgoUML, and of course - way back in the day - Rational Rose.  BoUML isn't as mature as Argo, and neither of them can compete with Rational for functionality.  BoUML's advantage is some slick that comes from being a new tool built on the shoulders of the other products on the market.

Still, I'm amazed at the amount of functionality and coverage of UML modeling that's available in an open source product.  I won't be using the code generation feature, arguably the highest-value it brings to the table; I'm just capturing a design.  But the product is exceptional in ease of use and has made UML 2.0 modeling a breeze.