The test is a lot harder than most people expect. When I took it, most of the people who were taking it had bachelors degrees in exercise science and were taking it for their third or fourth time. The anatomy questions are pretty tough. You really need to know your shit. The sample questions are a pretty good assessment of your skills, but just remember that the real test is LOOOONG.
I only used the home study guide and I past the test my first time. But, I had already been studying this stuff for the last 10 years when I took the test. A lot of people think they're just going to go in and master it, then find out they don't know near what they thought they did. All I can say is, make sure you really feel confident with your knowledge first. If you can go through the study guides and master the practice tests without much trouble, you're probably ready. If you can't, just wait until you can. Having to pay for retakes would suck.
The test is harder now than ever due to the national accreditation.