I find that the biggest advantage of Grails is that I don't have to care about the database anymore - the schema is automatically created / updated, and the persistence is largely done for me (no more writing SQL queries). This is a huge relief. The other thing that is rather nice is that once you settled on the templates for controllers and views, adding new domain objects is pretty fast. Although I suspect that you will do ongoing changes for your views at least, back-fitting them to the existing ones.
As for the IDE - it seems that IntelliJ is the best option, but I'm happy using Netbeans 6.5. I use MyEclipse for all other development, but Netbeans just has better Grails support now.
