The GMAT club tests, in my opinion, are absolutely mandatory for any candidate preparing for the GMAT. It helps practise timing and accuracy when it comes to solving Quant questions. The questions are quite close to the actual GMAT exam and are a good reflection of a possible score. The verbal is decent as well, though ideally I think you need more than 9 tests to get good amount of practice. There is a decent amount of explanation of the answers as well. Towards the end make sure to take a daily GMAT club quant test for atleast a week or two to get in to groove for the main exam.