I went to high school with the guy who wrote that post at facebook!
in terms of broker support, it works with IB... although it's only been tested with IB on windows. It also works with Assent... although assent is windows-only.
We have a small community of people who develop with it... also there are several firms and traders who use it every day... both for trading black/grey strategies and for some more mundane stuff like quote screens or risk management platforms. It comes with a bunch of applets but the library is multi-purpose and lets you build almost any trading "thingee" pretty quick.
hit me up on the developers mailing list if you want to help us get it running on mono...