Got my scores today...
First time taking it, got 5/6/6 45. The score is not ideal, but counts as passing for the USMC. Now for the gouge:
Preparation: long story short, I only studied for about a week and a half. You should probably not do this, and give yourself at least a month. If there's one book you're gonna get, get the Barron's - it covers the latest ASTB format, plus has a nice long summary of all the info you need to know for all the sections (unlike the ARCO). That said, you should at least download the ARCO and go through the practice tests until you can memorize them. Definitely make sure you download all the gouge on here.
I also wound up purchasing the FAA Pilot's Handbook (you can d/l it for free though) - this could be considered overkill for what you need to know for the Aviation section, but you're gonna have to learn it all eventually anyway, so why not?
Math: this was the section that killed me - not because it was hard, but because I simply forgot how to work many of the problems! The last time I took math was as a junior in HS - and I graduated from college 2 years ago (did I also mention I studied art?). Anyway, if you know basic HS-level math (algebra in particular), you should be fine. Time is always of the essence though, so you have to pace yourself. Practice as many math problems as you can possibly stomach (and then do some more)!
Reading: seemed a bit harder than the Barron's/ARCO one. The passages were more convoluted, and I always felt there was more than one good answer among the choices. There's no real way to prepare for this, so do your best.
Mechanical: this was the other section that got me, and I really wish I had prepared myself more for it. I'd say the Marine gouge and the book guides cover about 70% of what's on there. Make you sure you know all the basic physics principles COLD, as there were definitely a few curve balls here and there. BTW, don't waste your money on any Mechanical Aptitude books as those are garbage, and most of the stuff they cover does not apply to the ASTB.
Spatial: this was the easiest part for me, having played so many video games and flight sims over the years. That all said, it was slightly trickier than in all the guides (this statement holds true for every section of the actual exam) - some of differences b/w the pictures were VERY slight, and when you're rushing through them it's easy to make a mistake. Once again, proper pacing is key!
Aviation/Nautical: having read the Barron's, the Marine Gouge, parts of the FAA book and chapters 1 and 3 of the "boat page," I felt like I destroyed this section. There was maybe one question I was unsure about (can't recall what it is now), and overall it seemed much easier than the study guides. As has been mentioned a thousand times before on here, knowledge of the principles of flight is key. For everything else, just know the basic ideas and concepts - you don't have to know them in depth.
Supplemental: composite of all the sections. Most of these were on the difficulty level of the "easier" questions from each section.