These days I was trying Grails and I have to say that coding by convention with Groovy is very powerful and allows to create new web applications very quickly. It is very helpful when you create a new application because you can change the code without redeploying the whole application as you have to do with a WAR when you change a Java class. Grails has different environment and one of them is development, which is using embedded Jetty. The feature I really like is the integration with the Spring framework and especially the autowiring by name in Grails controllers.
Although, autowiring is not so new, it is not that easy to have it within an existing application which is using Spring and to integrate it with Groovy i.e. So, I saw in Grails a big big thing. The difference with the development environment and production is that in the dev environment, groovy files are recompiled and as soon as you change something, they are "redeployed". What a great feature, if only I could use it in another web or application container. Imagine that you have an application and that you could include a framework which is exposing all the beans that you need to your scripts, just by autowiring them.
Although, you can create a WAR file containing your Grails application and just deploy it to Tomcat for instance and you could still benefit of all the Grails feature but not of being able to recompile your "scripts". Yes, it is possible somehow but not that easily. You could still have your dev environment and tell Grails as soon as a script changes to publish it to the production environment, but that is not what I want.
Another possibility would be to use the features of the dev environment, is to integrate your existing application into Grails. You could expand all the sources and run it via Grails, I am saying that because I want to reuse all the configuration files (Spring, JSF, ...) of the existing application. But, really I do not appreciate that way.
Also something that attracted my attention were Grails plugins. There is a possibility to know that a plugin has changed and that event could be handled in your groovy code and used for reloading of the necessary scripts. More information could be found at http://docs.codehaus.org/display/GRAILS/Auto+Reloading+Plugins.
Well, I guess that for the needs I have, I do not need to use Grails and I could simply use Groovy with somehow the autowiring enabled in groovy scripts. Probably that would be enough. But the REST approach of Grails is also a nice feature that could just be reused without having your own code or dispatcher servlets.
So, go with Grails if you need to quickly develop a Java based web application. But I would like to be able to reuse the Grails development environment in another container too.