These tests are unbelievable. They are the closest questions I have gotten to actual GMAT questions. In fact, they are slightly tougher, but not too tough as to make the questions unhelpful.
These questions make you work for every answer, even the 500 level problems. These questions are excellent practice for the real exam. Using the quizzes feature, I was able to improve from a Q38 to Q43 to Q47/48. Without these tests, I would not have been able to improve my quant score as much as I have.
Thank you to the entire GMAT Club team, especially Bunuel who has the best/most simple answer explanations I have ever come across.