- It works pretty well, especially for basic uses (creating UML class diagrams)
- It's free software (in the sense of both freedom and money)
- It's a pure Java application, and works well on any platform supported by Java
ArgoUML's main drawback is the lack of an Undo feature, which is certainly a bit disconcerting. Web search hits of the ArgoUML development lists seem to indicate that this feature is in the works; if it gets done, then I my enthusiasm level would go from "pretty cool" to "KICK ASS". Even without Undo, it's still a good choice for occasional UML modeling needs.