Comment We actually looked at it (Score 5, Interesting) 57
Installed it, looked at the components.
While it's technically open-source, to make it usable beyond a simple proof of concept, you need to pay for a lot of "pro" stuff that requires a license.
I have no idea what it costs, but I doubt it will turn out cheaper as Microsoft.
Currently, it does not look like it's multi-tenant. So you need a full installation for every customer.