Over the years, I've had the opportunity to visit or consult with many large clients who have tried UML tools. Rather consistently, the developers eventually report, after trying the tool for some time, that it seems to "get in the way" more than help (versus simply a text-powerful IDE). This is not always trueI'm reporting averages. And, the experience of value seems to improve with each new generation of tools. All that said, the most consistent long-term report of UML tool value that I hear clients claim is their value for reverse engineering, as a
visualization learning aid to understand existing code. Generating UML package, class, and interaction diagrams from code and then viewing the diagrams on a monitor, or printing them on large plotter paper, seems to consistently be useful when developers want to "get their head around" a large code base. And I agree.
With time, as more UML tools become well-integrated with text-strong IDEs (such as Eclipse and Visual Studio), and their usability improves, I predict that there will be more consistent value reported in using the tools for both forward and round-trip engineering.