We are using grails + on the web layer + java with hibernate and spring on the service layer. It's the classic three layers (web, logic, data) where the web is grails and the logic is implemented in java. As is usual in java, we use bean objects that represents the data between different layers.
It works pretty well and it was the best solution for our case as the bean objects were already there, as well as the database structure. From our experience, I think grails has a great value as the web presentation layer, but I would stick with java to write the business rules and to persist the application data - as grails "is" java, all the grails-java integration is pretty straight-forward.
We use eclipse to develop the grails application and it's poor integration, as people said in here. But, as a suggestion from other developer, we run the grails application from the command-line and only use eclipse to save the source files, and it works pretty well, as the application is updated on the fly.
I yet don't feel comfortable for using grails in other places than in the presentation layer.
