You may not have had a chance to spot this on your travels but the Advanced Group Policy Planning Guide is available for download. from here.
This is great news as this gives some great new in-depth documentation on this superb product.
This white paper is an overview of Microsoft Advanced Group Policy Management (AGPM), and describes its benefits, how it works, and the next steps for evaluating it. AGPM helps you better manage Group Policy objects (GPOs) in your environment by providing change control, offline editing, and role-based delegation. AGPM is a key component of the Microsoft Desktop Optimization Pack (MDOP)."
Anecdotally I can say I am seeing a growing interest in this product.
Remember this forms part of the MDOP (Microsoft Desktop Optmization Pack)
I didn’t know just how simple and easy this was.  I was chatting to Jane who is onsite at a customer
For advanced group policy management there are plenty of very good working 3rd party applications. In our company we use a tool called active administrator.
This provides powerful active directory and group policy management allowing Resultant Set of Policies, GPO history and rollback, offline GPO repository. I can't say enough how much we like this tool.