I have used GMAT club tests and find it very similar to actual GMAT exam.
GMAT club tests have following advantages:
1. Huge repository of questions to prepare. All questions are of GMAT standard.
2. Test experience is similar to actual GMAT exam.
3. It is cheaper compared to other mock tests available.
4. It truly reflects where one stands. I have taken both GMATprep tests and GMAT club tests and find that my results are consistent which proves that GMAT club tests are true reflection of actual exam.
5. No of quant mock tests are specially noteworthy because no other test prep company provides such huge number of tests to practise.
6. Analysis of results are very helpful. It represents through graphs and pie charts. It indicates strong and weak areas. Avg time taken per question. Fastest attempted question. Slowest attempted question. It also provides accuracy percentages per difficulty level groups.
7. One can easily review wrongs attempts and also correct attempts. Explanation of each question's answer is detailed and elaborate. It helps most in strengthening concepts and approach to answer questions.