JRuby on Grails?There is a small ongoing debate about Ruby on Grails... the Grails founder, posted in thoughts on the topics too.
I don't know were Graeme takes it's numbers about Grails apps that are in production, but I guess that Ruby on Rails numbers are much higher. He argue that for a Java shop, Grails is much better framework than JRuby on Rails. His main argument to support this is that it will take less training. He seems to forget that to learn Grails (even if it's closer to Java than Ruby) also require some training. I would even add that learning the convention for framework like Rails or Grails is where most of the training is required... Learning the language is easier than learning the conventions.
In my point of view, JRuby on Rails has much more potential than Grails. First, Grails is playing catchup with Rails (still waiting for Migration in Grails). Second, Grails biggest strength reside on the integration layer between well know frameworks. However, these same frameworks are Java based and they come with the same limitations that the Java language has. Limitation that are not present in Rails because of the dynamic nature of the language it's build on.
Why would it be a good idea to run Rails on JRuby? Yes, integration with Java is a plus. But performance may be part of the answer. Ruby is not very fast (but it should get a lot faster very soon with the use of a Ruby VM), Java is quite fast. So running Rails on Java can make it faster. Even with Ruby VM coming along, JRuby may be faster because Java VM had a lot more work done in the VM performance than Ruby will have when it get it's own VM. If I'm able to run a Rails app in Tomcat or Weblogic, it will also have more appeal for existing Java shops. It could also give you the benefit of having access to existing Java libraries that have no equivalent in the Ruby world.
Instead of working on making JRuby as a language to run on top of Grails, if integration with existing libraries (like Hibernate) is desired, I think that Integrating the existing library in Rails would be much better. One could easily write a simple layer around Hibernate so one could have model objects inheriting something like HibernateRecord instead of ActiveRecord. The dynamic nature of Ruby could make this HibernateRecord behave the same way as an ActiveRecord. As for presentation framework, I don't think that component-based framework are such a good idea anymore. So there is no need to ever want to integrate them in Rails.
Graeme argue that Sun should not longer put resources in Phobos because the project is unpopular... I could argue that same thing about Grails... Compared to Rails, Grails looks like the unpopular guy in the schoolyard. But I won't do that... While I personally don't see why would someone would choose Grails over Rails, I thing that competition between frameworks is good. And when Grails will start to innovate (instead of playing catch-up) then it may become really interesting. The unpopular guy of today can always become the most popular guy of tomorrow (this is the Ruby and Java story).