It's really a test no one should fail. The old exams have several repeated questions and all the information is in the MPEP, you just have to know how t find it. I worked as an examiner, so I had a big head start, the most useful studying I did was with old exams and some study materials. I took a class and it was a waste, but it probably would not have been if I had not been an examiner.
You'll know before you take the exam whether you are going to pass or not. You should be in the 80s on the practice exams I believe.
Like Chemdr, I left knowing I had passed, there's not a big time constraint, so I had plenty of time to look up answers I didn't know outright. I would guess I scored in the 90s and I think anyone can with sufficient preparation.