I have seen some great work done with activity diagrams and use cases on smallish systems. However, it has always been my experience that mixing the UML functional modeling techniques with larger scale efforts spells big-time trouble. Proper analysis requires postponement of implementation aspects until the appropriate time. Use cases, by their very nature, are implementation oriented; they force the analyst think of solutions too soon.
Remember, agile does not propose any new modeling techniques, and proper use of modeling is key to success. Don't let the latest buzz word cause undue confusion. I would focus on gaining a clear understanding of what the important differences are between functional modeling techniques.
Tony