For first-timers, the test takes approximately three hours and is divided into Job Knowledge(JK) , English Expression(EE), Biographical(BIO), and an essay. You will hear a lot of different opinions on the test, but in my opinion you can prepare for it. The JK section is very broad and not very deep and you could study for years and not see one question from your studies. That being said I found the Cliff Notes Quick Review Books on American History I and II, American History, and Management to be very useful for my preparation and I would consider myself to be a bit of a history buff and news junkie. My normal routine was to read the NY Times and the WSJ, but I added the Washington Post, the Economist and a Middle Eastern newspaper to my daily reading ritual.
The EE section will look familiar to you if you have taken college or graduate school level entrance exams. It is your typical take a really awkward sentence and make it less awkward. I have never done particularly well on those sorts of tests, but did brush up on my grammar rules and if I have to take the test again will spend more time on this section as a way to raise my overall score.
The BIO section is the section that mystifies most test takers and by far has the biggest delta in terms of test scores. For those long in the tooth like me don't be fooled that a good BIO score equals experience, and younger test takers do not let a lack of traditional experience hold you back from taking the test. The BIO section seems to be more about how you answer the questions in terms of how you fit a State Department profile rather how many languages you speak, have you lived abroad or not, etc. So don't fret if you do not have a lot of international experience. Remember you are being hired as a generalist.
Your essay will only be graded if the combination of your JK, EE, BIO scores reaches the passing mark. The current passing score is 154. I don't know if that number will ever change and it appears that the State Department does not change the pass/fail mark as a means to adjust people moving through the pipeline.
The essay is graded on a scale of 1-6 by two graders and the individual scores are combined to give you an overall score. Currently you need a score of 6 to pass and from most of the self-reported scores on the yahoo group, 6 seems to be the most common result. The essay is definitely a time crunch and I know that I was editing it down to the final seconds. My best advice for the essay is to sit down at your computer with the prompts, turn off your spell checker, set your watch for thirty minutes and just practice. I think I practiced 4-5 essays in my last two weeks before the test and did fine.
Now the score you receive is based on a T Score and most people want to know what the max and min is and what the score itself means. From what I remember from my time taking statistics in college and in grad school is that you can not read too much into your score since you are being compared against your testing cohort and it will depend on who got certain questions right or wrong.
Going into the test I felt that I would do much better than most on the BIO section, as good or slightly better than most on the JK section, and slightly below average on the EE section. I just about got it right except for the BIO section. While I did pretty well on it I was under the impression that traditional experience was what mattered most and I don't think that's the case.
My last bit of advice is not to listen to too much advice! Whether you pass or fail no one really knows why, so take all of the advice(most of it well intentioned) you find out there with a grain of salt.