I actually have much more experience with plantuml (we actually have a plugin which generates diagrams for our jekyll/asciidoc docs site: https://github.com/FusionAuth/fusionauth-site/blob/master/si... ), but I have recently started using mermaid for a project and like it so far.
I'm surprised that mermaid is gaining so much traction in comparison to plantuml which is much more powerful. I guess it's the ease of integration - you don't need to setup java backend just to render diagrams.
I'm still choosing plantuml over mermaid whenever I can. Mermaid has too many quirks and limitations.
