When to Use Activity Diagrams

The great strength of activity diagrams lies in the fact that they support and encourage parallel behavior. This makes them a great tool for work flow and process modeling, and indeed much of the push in UML 2 has come from people involved in work flow.

You can also use an activity diagram as a UML-compliant flowchart. Although this allows you to do flowcharts in a way that sticks with the UML, it's hardly very exciting. In principle, you can take advantages of the forks and joins to describe parallel algorithms for concurrent programs. Although I don't travel in concurrent circles that much, I haven't seen much evidence of people using them there. I think the reason is that most of the complexity of concurrent programming is in avoiding contention on data, and activity diagrams don't help much with that.

The main strength of doing this may come with people using UML as a programming language. In this case, activity diagrams represent an important technique to represent behavioral logic.

I've often seen activity diagrams used to describe a use case. The danger of this approach is that often, domain experts don't follow them easily. If so, you'd be better off with the usual textual form.