In my experience, rewriting for JRuby is as much work as re-writing in a new language. Last I looked was a year or two ago, so it may have all changed, but it's not as simple as just moving your code over. Many really important gems don't work, or don't work properly.
That used to be the case, but a lot has happened in the last few years. Most of the time you'll be able to move a large Rails application to JRuby without any changes at all, except adding a Warbler config for deployment.
We use JRuby for a one of our backend applications in production, but we develop them with plain MRI Ruby. Only real problem we've run in to was some of our badly performing (badly written) parsing code that ran even worse under JRuby.
The only gems that do not work are those which are not pure ruby. For the popular gems I think there have been rewrites. I haven't looked into JRuby but it has changed quite a bit in the last year from what I read
I don't think that's true. There are plenty of threadsafe gems with C extensions, and plenty of non-threadsafe pure ruby gems that mutate class/class-instance variable willy-nilly.
