I have long observed Grails passively, rejoiced that there is now a web framework in Java that does not require jumping through hoops to get a basic setup working. Free time has been a bit of a constraint through 2008, so I never got enough time to spend exploring Grails.
When I heard about the release of Grails 1.1 Beta 2, around Christmas time, I decided I had enough of a free time during the holidays to actually start learning Grails.
A good tutorial that I started following is a Grails Web Album tutorial by John Leach - It is a fairly complex application about creating an online photo album. I was easily able to follow the tutorial and at the end was amazed by how simple it was to create a working application. A great thing about the tutorial is that an equivalent Rails version is also available at John Leach's site, that allowed me to compare and contrast Rails with Grails. I am already fairly proficient in Rails, so one thing that stood out in Grails for me was that specifically for the Web Album application, all the image manipulation libraries were available right within Java, whereas the Rails version required using RMagick, which is fairly difficult to configure on a Windows XP box.
My conclusion is fairly simple - Both Grails and Rails are compelling Web frameworks, both have a fairly flat learning curve and the underlying languages(Groovy, Ruby) are delightful and fun to learn and use. I very much intend to continue exploring both Grails and Rails.