Good Range of topics and questions.

Gmat club tests are very reliable tests. In my experience, they are cover important topics very well. The questions are similar to the ones which you would see in gmat preps or the real exam. Quant tests are very useful to learn/approach advanced questions and to refine one's skills and pacing.
Quizzes and practice mode are great help where you can questions topic wise and identify your strength and weakness. It just makes your preparation for quant a bit more organised. It makes tracking very easy.
They also provide difficulty level of questions where you went wrong in Results. Would like to very much recommend the gmatclub tests for one's preparation towards the GMAT.