I would often use ArcGIS Diagrammer for bigger projects, ones which would require documentation. I’ve never actually included any of my diagrams along with the project documentation, mostly because I was never asked for it.
Once I had worked with it long enough, I started to notice that it could be quite cumbersome and unintuitive at times. If something did not work as expected, I would first carry out the action manually in a file gdb, export the xml and look at it in Diagrammer to see the workflow.
Eventually, I just ended up going back to creating my templates in a template gdb. If I ever need a snapshot of my gdb in a Visio style, I will definitely open it up again, but it just doesn’t fit into my daily workflow.
What I am a bit irked about is the fact that ESRI never bothered to come up with a more official version of this product.