I passed my PMP today on the first try. I read the PMPBOK once, viewed all of the videos relating to the PMBOK only, prepped my own study guide and took some quizzes. On my first prep exam, I got a 80.5%. Feeling confident, I registered for the exam. Within a week of registering, I took 4 more prep exams getting a 75%, 79.5%, 79% and a 72%. I still confident enough to take the exam today, on which I scored a "Above Target" on 4 of the Processes and a "Target" score on the 5th process, so I feel I must have received at least an 85% on the real exam - I never scored that high on the prep exams. I personally feel you don't have to wait to score 80% on all 8 exams. I only took 5. If you're getting anywhere between 75% - 80%, take the exam! Hope this helps.
