I'm using JRuby in the code I'm developing for my startup, in order to access Java libs I need. The biggest issue has been that it doesn't support Ruby 2.0 yet, which is merely aesthetic at this point for me. JRuby hasn't given me any grief since I got it going.
It's also stable, even though it tends to lag a little behind cruby in some features. The 2.0.0 feature set is not yet implemented fully.