I used the one called TEAS, it was $9.99. It wasn't really worth it. The questions were far easier than the actual exam. The only thing it was good for was to keep me occupied with something more constructive than iPhone games.
That means that this app actually helps! When I took my TEAS I only used the ATI book and passed with high score. My daughter just took the exam and passed. She used the Kaplan book and she said that this book helps the most because the questions on it are harder than those of the actual test. I took my TEAS three years ago; I am done with school and passed my NCLEX RN as well. I did not buy the ATI book this time for my daughter. I only bought the Kaplan and it helped her pass. You can rip the individual tests so you can take them on-the-go if carrying the book is somewhat inconvenient.
I wish you luck!