In google-groups was one entry where someone told that he wrote a facebook-app, with 1.000.000 Hits per month, so it shows that you can use it in great projects.
I'm unsure what exactly you want.
About grails and maven: Play and Maven is difficult too, at least there are a lot of mails in the google-group about it. However, since 1.2, there is better Maven support. You can add Maven dependencies directly from conf/dependencies.yml. There are probably some restrictions, but it works well in general.
