Constant Contact uses Rails on jruby pretty extensively, because of improved performance and JVM integration. There was also a talk at Boston's Wicked Good Rub

Constant Contact uses Rails on jruby pretty extensively, because of improved performance and JVM integration. There was also a talk at Boston's Wicked Good Ruby conference of a case study of a large Struts 2 app getting extended in Rails on JRuby. (I asked about Grails as an alternative; not only was Groovy more painful than Ruby to work in, but Grails itself, in the speaker's experience, was more difficult to integrate into an existing application stack. Which is something the Grails folks might want to look into, if it's correct...)