i personally liked the OAT achiever. maybe i'm pessimistic? i only took 2 practice tests on it, and i was completely devastated from those scores... which made me ECSTATIC when i got my actual test scores, which were much much muuuuuch higher! the achiever questions are extremely hard, but i think it helps scare you into preparing more. or sometimes, too much - but what is too much for the OATs?
i like the format, as it is very similar to the actual, and you do get a good grasp of how long it's going to take you to get through the test.
i studied out of the Kaplan OAT book, as well as the Kaplan MCAT comp review book. the mcat book was very helpful, as it was condensed into what you needed to know. much easier than re-reading all of my old bio and chem texts. all in all, all the practice exams i took were MUCH harder than the actual, so i think i overstudied the hard stuff, and could have focused more on the easy stuff. eh. i'm happy in the end. 😛