Who is using this? I have only seen plantuml written about on blogs, never actually in use.

I’ve used it for a long time; even if I don’t end up sharing with others it can help me quickly visualize things (the ease of change often makes better than pen and paper). At one of my jobs we had an architect join and he was amazing; I really respected him. He used PlantUML and would open it up and start writing sequence diagrams with us. He would make sure complex flows that were actively being implemented were kept up to date and they were useful references.