I'm nearly to the point where I DNF'd the book the last time, so it seems a good place for an assessment of the book.
As I noted in a previous post, this is an incredibly detailed book. Fortunately the details aren't used to provide a day-by-day account of Wilhelm's life, but instead inform Röhl's analysis of it. He is adopting an almost forensic approach to examining Wlhelm's development, though at times it's more focused on how the prince might have turned out differently than on why he turned out the way he did. The result helps affix some of the blame on the people in his life (most notably his parents and his tutor), but I can't help but feel Röhl's a little too invested in the blame game.
Because of his focus, Röhl's book has some elements of psychobiography in it. I've never been a fan of the form (psychology is inexact enough when you're dealing with a living patient you can interrogate; to do it with the fixed records of the dead seems too much like reading tea leaves), but Röhl doesn't take it so far that I have to set it aside. It helps that his insights are usually perceptive and always well-informed. I don't always agree with his conclusions, but I have enormous respect for them.