I have taken the tests of all the major online GMAT test service providers, and GMATCLUB is definitely one of the best test series. The question pool is good. You get to work on questions that are tougher than the actual exam.
I used the test series to work on my verbal timing. I tried to solve the tests in 65mins - 70mins to account for the variabilites in actual exam. I hope that the actual test questions are no more challenging than the GMAT Club ones.
I will strongly suggest everyone to give the tests a shot before you appear for your actual exam.