GMAT Club Tests offer hard Quant CAT tests and a smaller number of verbal tests (over 1,100 quant questions and 388 verbal questions). The questions are generally above average difficulty though some easier ones have been introduced as well. You can see exact distribution of questions here.CATS provided here have really helped me improve my quant score .Also it has boosted my confidence level to attempt the question.The quant part of the CAT have the best questions that you will find anywhere. And it's not just the questions themselves, the explanations provided also help you uncover many shortcuts and help strengthen your core concepts. Admittedly, I found them to be a little harder than the GMAT quant questions, but that should help you be better prepared for the final test. And also, you can take the quant section or verbal section one at a time, which gives great flexibility when you are hard pressed for time. The discussion in the forum is also very helpful to understand the questions. Talking about verbal, CATS here are also good , they have helped me improve my timing .
According to me GMAT Club should incorporate more Verbal CATS , so people like me who face difficulty in completing the test in stipulated time , get better hands on.
The summary is these tests are value for money.