GMAT Club tests is the must have product for anyone who is aiming 720+, and especially for the person who wishes to have magical Q51 in its Quant section.
The tests not only test your comprehension skills but also equip you with critical thinking skills wherein you have to put in an extra effort and attention for answering the question correct in time.
However, there are some flaws in the GMATClub verbal tests as I have always received poor scores in Verbal in GMATClub, but that could be because it is difficult for any test maker to emulate the algorithm of GMAC accurately. Moreover, any testmaker in this world is unable to create an exact replica of GMAC.
Still, I believe GMATclub tests are a must have product for all the people who wishes to hit Q51