We're using JRuby in some core projects and it's great.
From my experience (moved 2 high load projects to Jruby) transition to JRuby from Ruby is not just changing Ruby version. But often you spend around 1-2 weeks to move medium size project on it, change some gems and configure Java options to not have out of memory errors.
So it works fine, I'd not say that it works much faster than latest Ruby. But I think that main reasons why you should switch are multithreading and Java libs. We switched because of we was need to use latest Java libs for Kafka.
But the main disadvantage is that sometimes when you need to deal with Java objects you need to think about object data type casting (from Ruby object to Java and vice versa). And this is
extra actions, extra memory usage.
I'm very happy to see JRuby 9000 and hope we'll upgrade soon.