To be honest, the tests are just like any other CATs ,but the only distinguished features are -
1. more 700+ level questions
2. more GMAT like experience
I got the subscription for gmatclub tests from my eGMAT verbal Live prep course. It was a good experience. To be honest, my quants skills got improved and now I am more confident to score Q50+. These questions really reflect the upon the concepts required 700+ quants questions. On verbal side, I feel that the RC is proper and is as per the GMAT standards.
The only negative is that you need to have a proper low latency internet connection to have a satisfactory experience.