It is a formidable exam which consists of a difficult written exam followed many months later by a very difficult and intimidating oral exam.
In the late 90's the pass rate for the written dipped to 59% if I recall correctly. If you look at the pass rate for people retaking the exam, the pass rate is dismal ( a few years back the retakers passed an average of ~33% of the time)
The most current numbers I have seen are from 2006
88% of 1st time takers passed the written and 76% of 1st time takers passed the orals. For the orals, the numbers over the last 5 years have ranged from 71% to 82%(2005). For the written, the trend has generally been upward over the past 5 years. It was 71% in 2002 for first time takers.
For retakers in 2006, approximately 42% passed. If you combine retakers with 1st time takers, you get an overall pass rate of ~74% for 2006.
For the oral exam, retakers hit around 58% and combining both retakers and 1st time takers gives you an overall pass rate of ~72%
My gut feeling is that these numbers will go up as brighter people(better test takers??) choose our specialty.
The moral of the story is, study and pass the exam on the first try. You will be out in a busy practice working, perhaps, harder than you did in residency and you will have little time to prepare for retaking the exam. This is easily seen in the stats for the retakers. You would be surprised how much you can forget just being one year out of residency. You are learning the practical stuff, but forgetting the basic sience stuff that they will test you over.
Pass it the first time!