Looks promising! The multi-org feature is interesting, and not something I see very often with these types of solutions. Is the use-case that I host and manage it, and onboard my clients as users in their own orgs?

Yes, exactly that.

Personally I have several small (tiny really) companies where employees wear a few hats in them, this setup allows me to add people to org (and email domain) A & B while not getting C.

It is _not_ setup at all for billing or anything like that though, but that would be a easy enough feature to add