Good tests !

I believe that the GMAT club tests are reflective of the level of an individual's preparation. Although at times the GMATClub tests can give you a few 500 level questions in a row irrespective of your performance up until that point, the overall test scores seem accurate.
The GmatClub tests have a good mix of a variety of questions and writing these tests will help you assess your strengths and weaknesses better.
I do feel that a few of the English questions could be explained in greater detail, though most of the explanations are correct and to the point.
Overall the Maths tests are amazing and the English tests are good. I strongly recommend writing the GMATClub tests before appearing for the main exam.
Good luck :)