I liked the Boson practice tests, mostly for the detailed explanations of both the correct and incorrect answers.
The actual test is about being able to apply logic and reasoning to your knowledge of the material. I found most practice tests just are just Q & A about facts. Yes, you need to know those facts, but the real magic comes from first understanding what is being asked, and then applying your knowledge to come up with the most logical answer.
The Sybex tests were good for demonstrating your knowledge.
Just to be clear I have sat and passed the exam and I promise you there isn't a test engine that resembles the exam, so it's really ensuring you understand the concepts, not just memorised them and questions. You WILL FAIL if you do that.
Read the material, understand the concepts, use the Sybex test engine that comes with the book and your good to go.