Having passed the PMI-ACP exam yesterday with Proficient level on both domains, I thought I should share my experience with all of you.
I used the Agile PrepCast, Mike Griffith's book, and the exam simulator from RMC. I watched all lessons in the PrepCast and taking notes for every one of them. Then I moved to Mike Griffiths' book, and read it once from cover to cover. I tried one mock exam to find out how prepared I was (I scored about 73%). Then I read the book again, reading one chapter and then taking a mock exam choosing the questions specific for that chapter (domain). When I finished the book, I took a couple of mock exams and my score was above 90%.
I would advise people to focus on Scrum, XP, and Lean. Make sure you know the Manifesto Values and Principles.