I think that peoples opinions of the ASTB study guides are similar to peoples opinions of boot brands. It seems like you get people posting more negative info about some other brand than positive. The study guides shouldnt really be teaching you the stuff, just to help review and get you thinking along the lines of the test. I bought the ARCO book and I looked at some other brand (sorry, cant remember the name) and they appeared to be the same for me.
I would reccommend picking up a copy of one of the major brands and read around on this site. If there are areas that you are really suffering on and the book doesnt help, get out a book on that subject from a library or post a specific question on here. Good luck with the test.