As a counterpoint, not every Ruby app is a Rails app. I've been using JRuby more and more lately for various text analysis projects. Calculating the mutual information over a huge set of tokens, for instance, was roughly 10 times faster in JRuby... And of course didn't suffer from the Unicode issues I was running into with Ruby (1.8). Regardless of whether or not it's suitable for Rails, it's absolutely suitable for other projects.