@kris: It’s better than nothing, but there are some usability issues.
Namely, the “auto-sizing” needlessly creates vertical scrollbars, when it’s clear that the entire chart can fit without “cropping” it from the top/bottom. (I’m trying to find out why it does that.)
See this post for an illustration.
There’s also this annoying quirk where it “intelligently” tries to move your text cursor around as you’re typing. Unlike other “dumb” text fields, it feels like the mermaid plugin wants to help you out… a bit too much.
EDIT: Another issue is when someone tries to “quote” your post that contains a chart. 