I didn't use it a lot, but it definitely was invaluable as a preparation. I have an MS in math and there was something I was misunderstanding in the practice tests that made me score in the 80s (mixed numbers, I was treating them by multiplying, we never use mixed numbers in upper level math so my brain is trained to multiply!). Thanks to the study guide, I scored in the 99th percentile. I actually wish I had spent more time looking at the English section because it was my lowest score.
Good luck to all who still have to take the TEAS!