We're expecting beta 2 to be release in the next month or so, and the final MultiChain 1.0 release over the summer. In the meantime you can certainly build on beta 1 with the knowledge that nothing in the API or parameters will change before 1.0. It's also very stable and we've only had one crash report due to long stream item keys, which we've already fixed internally.
To answer your other question, there are a handful of projects already in production on MultiChain, and we expect many more to be coming online in the next 12 months.
