I think I’ve found the UML tool I’ve been looking for. StarUML, still in beta, is shaping up to be a great UML 2.x tool that has a beautiful, minimalist interface that lets me focus more on system analysis & design than using a tool.
Keynote & Sketch are great tools that I’ve used for simple system-diagraming. But the resulting diagrams are static, they’re just images describing systems. Maintaining them can quickly get difficult, and there’s no embedded logic in the diagrams. By using a true UML tool, logic is embedded in the diagram, dramatically increasing their value, and they are much easier to maintain.