The exam simulator here has the best mock exams I've found. I tried a handful of others but none of them were as close to the actual exam as the ones here. It is definitely worth the money, in my opinion. I took all of the full 4-hour mock exams here and reviewed all of my incorrect answers to understand why I got them wrong. That's the great thing about the exam simulator here - it explains all of the answers so you can better understand the material. If you spend a few weeks doing these things (assuming you've already read the PMBOK and taken a class), and get to the point where you are scoring over 85% on the exam simulator, you will be well prepared.