The Grails team has started a Google Plus discussion to gather feedback from the community on how their upgrade to Grails 2.0 went.
To sum it up, small applications can be upgraded easily to Grails 2.0, but for large application the upgrade is quite painful.
Peter Ledbrook has published a Guide on how to upgrade an existing Grails application to version 2, summarizing the pitfalls that have been found by the Grails team and the community. This guide will be updated when the Grails team has gathered further feedback from Grails developers.
Some hours ago, the Grails team has published an advice on Google Plus: If you want to migrate your tests to the new unit testing capabilities of Grails 2.0, wait for the next bugfix release (which is due to some issues found):
If you’re upgrading to Grails 2 and you want to migrate your tests, it’s probably worth waiting for 2.0.1. Several important fixes related to unit tests coming in that version.