For me it's always the triple-whammy of being a bit (or a lot) slower, needing vastly more memory, and having astronomical startup times. Not exactly an appealing combination.
If you're seeing a situation where you need vastly more memory with JRuby, please file it. While there is a higher base memory footprint due to the JVM being booted, I've found in practice it's much better on RAM than MRI. We saved a lot of money on EC2 by switching to JRuby because servicing a new request with a thread was a lot cheaper than forking a process running Rails.