I use Excalidraw because I like the hand-drawn look when planning/sketching things out.

If/when I make the jump to actual documentation I use Mermaid so I can keep everything tidy under source control.

+1 for Excalidraw, especially regarding the "multiplayer" aspect.

In general, if sketching it out, whether alone or with multiple people, using something simple I can click, drag & label is more than enough and helps me keep the flow. (I would stick to actually drawing - either on a whiteboard or something digital - but my handwriting is absolutely atrocious)

As soon as I'm done sketching and move over to actually documenting a concrete solution, something code-based like Mermaid - as you said - is much nicer. But both are entirely different processes for me.

In Mermaid, while I like it a lot, you lose control of layout positioning. Does the auto-layout position large diagrams (many shapes + connectors) appropriately?

The Excalidraw export format is JSON, so you might use that in source control just as well. Or include the format in PNG or SVG export (which draw.io supports also).

Excalidraw is very nice in it’s simplicity. A rectangle can do a lot there: a textbox, a window, a mobile phone.

draw.io (now called diagrams.net) has a sketch style that mimics xkcd / Excalidraw. You can try it here:

https://app.diagrams.net/

There is even a desktop app available (but the webapp works offline too):

https://www.drawio.com/blog/diagrams-offline

The way most people use draw.io is using cloud storage integration like Google drive, hence versioning is not possible. So far I never seen people put the draw.io produced file to Git. I'm using PlantUML in some of my projects, it solved the versioning issue. However it's lacking of lots of draw.io or even inkscape editor capability.

There's a plugin for VSCode that brings draw.io editor for local .drawio files. IMO it's the best of both worlds: nice editor and git version control.

Also this plugin works fine in github.dev so it's like a better version of draw.io official site.

I do. What I like the most is the ability to include the diagram source into PNG metadata. The viewers don't need any renderer, yet it can be readily edited.

Connect it to GitHub then you have commits for every change