I picked up a copy of The Definitive Guide to Grails this week. I'm most of the way through it, and I'm impressed. The Grails team has definitely done a lot of work. As a result I've started to dig into the code, and I'm getting a lot of ideas for Groovy Works.
Overall I highly recommend getting a copy of the book, whether you use Grails or not. It is well worth a look.
I'll post more thoughts when I finish the book.