I'm using it for a development server at the moment - we are planning a major upgrade of the university system from 1.9-2 in the summer (It will probably be 2.3 by the time we go live as I'm looking at the major changes that are being developed for the assignments module).
At the moment I've had no trouble at all with any of the modules I have wanted/needed to install from the plugins database or with any of the modifications I've made, so as far as I can see, 2.2 is stable. The only plugin I've tried that I haven't had complete success with yet is ILP and that is currently being looked at by James so I hope to have that working shortly as well.
That said, the development server is going out for wider testing next week, so if I have any issues, I will report back.
Richard