I feel that subscribing Gmat Club Tests was helpful to practice GMAT. Moreover, it is worth to buy.
I can even practice with type question that I felt weak on it, such as 600 questions or the hard 700 questions. I can also practice with only several questions, instead of the full CAT, in order to get usual of CAT.
Unfortunately, I did not have much time to practice. I will buy again and practice many questions as I can.
If I can give some suggestion, It will be more helpful if I can get an immediate response from an expert about the answer that I can not understand the reason.