comprehensive tests for all levels

The number of questions that GMAT club tests provide is far more than any other tests. They give you the option to create custom quizzes based o difficulty, topics and section of gmat.
A very good practicing resource and helps you gauge your performance in the real exam.
Quants is little tough bu it helps you think beyond and enhance your practice.
Verbal section is good in terms of comparison with other products but again the best source is official guide. However they are very close in terms of question format and difficulty level comparison.
Explanation of each question is given by experts and discussion forum helps you clarify your doubt.