[ghostganz]

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.