I took the ASTB on Monday. I scheduled it about 6 weeks prior, but in most cases I doubt that would be necessary. I have a very full schedule, and it was the first week day I had time to do it on. I am not sure if what I have to say can really add to the great info that people have already contributed to this thread, but FWIW, my impression/experience:
First of all, I took Form 5. It was the computerized version. After reading others' opinions on it, I was a little apprehensive about this mode of examination, but after taking it, I would say that it made life easier in the long run. I did wear a watch because I didn't know what the timer on the computer would be like, or if I would even have one. I was glad to find out that my watch wouldn't be necessary. It also really won't let you do something you don't want to do. Don't be nervous about hitting a button and having it submit a section against your will or anything. Warnings and notices abound, and you won't begin or end a section without being told explicitly by the computer.
To be completely and utterly honest, I didn't study nearly as much as I probably should have, but I ended up doing much better than I thought I would. I hadn't really had much communication with my recruiter, and this was the first time we met face-to-face. He suggested that I do some very light studying, and then come in and take the exam to see where I was.
The Test:
I am a terrible test taker, and usually don't fare well in standardized test situations. I am sure some of y'all can relate. I can do it in class just fine, I can do the homework just fine, I can get all of the answers on the study guide just fine, but when it's time to make the donuts, I have a tendency to just lock up. So for any of you who haven't taken the test, and are extremely nervous about the entire thing, I have one word for you: RELAX. It was way easier than I had anticipated, and from what little I had to do with the books, [Peterson's, ARCO, Marine Gouge] I felt that they do an excellent job cluing you into what the real test will be like. I will even go so far as to say that the math and mechanical comprehension sections of those seemed to have much trickier problems than what I experienced with the real deal. I do have my PPL, and do work in the aviation industry, so the questions asked in the aviation sections were not any real issue for me, though if you don't have any aviation background, I would highly suggest you invest in the materials suggested in other posts. On the other hand, I know a lot less about boats than I thought I did. The reading section is about what most people would expect. Read the paragraphs, choose the right answer. The same can be said for the spatial appreciation. I would definitely practice that as much as you can until you are comfortable with it. I was able to complete every section with some time to spare.
Once I finished they printed off two copies of my results, handed one to me, and had me sit in waiting area while they went and tracked down a recruiter for me. The recruiter I had been corresponding with [and by that I mean I had talked to him on the phone once, and setup my ASTB via text message.] was out for the week, but they had another guy who was willing to talk to me. He asked me how I thought I did, and told me that based on the information and numbers he had, he thought I would be pretty competitive with my scores, and told me that a retest wouldn't be necessary for now. So yeah, I filled out a bunch of paperwork for MEPS and will be going next week. I plan on graduating this December, and it looks like we will be shooting for the board this summer!
Study materials that I would recommend would be what everyone else already has. If you are exceptionally nervous, start with the marine gouge first, as it does a very good job of presenting some of the same information, but in seemingly easier terms.
All of the online stuff I glanced at I found through this website:
http://recruitingcommand.com/texasnavy/2012/08/02/astb-gouge-2/
Obviously the ARCO book. I am pretty sure you can download it from somewhere, but I have the hard copy.
This PDF that the Navy has on their website is very, very useful. Especially the spatial appreciation. Very. Useful.
http://www.med.navy.mil/sites/nmotc/nami/Pages/ASTBOverview.aspx
http://www.med.navy.mil/sites/nmotc/nami/Documents/ASTB_SampleQuestions JUL 2011.pdf
Hope I didn't bore anyone to death.
Also, there is a simulation test I found online that I used as my baseline as far as meeting time requirements. It looks like it is somewhat restrictive about who may use the material when you register. "Open to members of the armed forces and their families." I have no idea how they would enforce this, but my brother is a Marine, so I went ahead and created an account and took the test.
http://www.nelnetsolutions.com/dod/DODHubPage.aspx?sponsor=12894&HubPage=15150