I have considered it deeply. Many of our enterprise systems are Java-based, so JRuby's ability to call Java methods directly without implanting services would be a huge pragmatic boon. But every time I consider that I have to balance it against being able to use a constellation of gems in Rails. For example, nokogiri (libxml) is used in a ton of stuff. Sure there are Java equivalents, but that isn't the issue. Rather I would have to reimplement or find replacements for everything we use that depends on that without introducing side-effects. In my experience that's really hard unless it's a toy application without many dependencies. (Read: opposite of most enterprise Ruby apps)

Also, as a gem author I've tried to support jruby along with MRI, but it requires jumping through a lot more hoops. Of course any cross-platform code requires more work to support, but it's a pressure on small devs. If no one needs it, no one helps support it, so then it becomes another gem that is effectively MRI-only. Kind of a viscous cycle.