The GMAT Club Tests are a must to achieve a great score in the GMAT, especially math. Since I am a non-native English speaker, when I started my GMAT preparation, my focus was on the verbal part; however, a month before my test, I realized that my math was really bad because it has been a long time since I've practiced it. So I started studying using only the GMAT Club Tests and the posts that are related to the tests (you can also find the theory in the posts). Before my GMAT exam, I managed to practice approximately 12 math CATs, which helped me familiarize with the topics, relearn theory and get used to the timing of the test. Unfortunately, I only got 48 in the real test, but that's because of some interruptions during the test and a bad timing strategy (I had to guess the last 4 questions!). I'm confident that the scores in these CATs are quite similar to the real GMAT. In the last CATs that I took, I was scoring from 48 to 50 so scoring 48 in the real test was OK, considering that some external factors prejudicated my performance. The verbal tests were also quite good; however, the quality of the answers was not as good as that of the math. To sum it up, you need these test in your life as soon as possible! Since they are so many, you can even use 4 of them to have wide diagnostic of your level in both math and verbal. Good luck!