You've taken the test already, but I'll respond anyways since I'm sure others who haven't are still looking!
I got 93.5% composite (99% nationwide/program). I didn't use the ATI study guide - I used a generic health career entrance exams study guide (kaplan, I think). I thought it was a huge help, mainly in giving me practice with the math. The math isn't particularly difficult, but if you're going into it cold with no review, you're likely to run up against time problems or make small mistakes.
Realistically, any SAT/ACT prep book is going to provide adequate prep for most of the test if you have one sitting around already. I don't think you really need the ATI study guide specifically, though, not having looked at it, I could be wrong. I think the probably-kaplan study guide I used was oriented towards the HESI, which, from what I gather, has a much harder reading comprehension section. The RC section of the nursing exam study guide was totally hard and scared the heck out of me, because that's usually the area where I score the highest, and I didn't do particularly well. The actual questions on the TEAS were much more straightforward.
Be familiar with the metric system and conversions. Especially metric. These don't make up a huge portion of the test, but every little bit helps, and they aren't hard to memorize.
I have a book called Get Ready for A&P, and found the chemistry and biology review in that to be the perfect depth. A basic chemistry book aimed at middle or high schoolers would probably work, too. The earth science stuff was pretty much sink or swim. The physics seemed liked stuff that could be figured out by deduction, even for someone not particularly familiar with physics (I took two years of physics a decade ago, so it wasn't unfamiliar to me).
During the test.. if you're taking the computerized version, don't skip questions! If you're stuck, make the best guess you can, make a note on the scratch paper, and come back to it. On our computers, going back was a very time consuming process - you basically had to hit the back button as many times as necessary to get to the right question, and the pages were slow to load. It wasn't a problem for me, but enough people run into time problems that it seems sensible to assume you won't have time to go back.