GMAT Club Test

The Quant Tests are very helpful! I find the questions are very well designed to touch upon the fundamentals that are essential for a high score on GMAT. The subject ranges tested are also pretty wide ranged. More importantly, the questions are very tricky; so they help get you very attentive and cautious. The biggest advantage of solving the question sets is that you will be able to minimize silly mistakes by leaps and bounds. These silly mistakes cost a big chunk of the overall score. The other half of this amazing story is the excellent pool of explanations for each question. Some of the explanations are very elaborate and also, go beyond the concepts necessary for the question in hand and rather discuss at length covering similar concepts. These question sets are like a learning platform in gradual stages. An amazing tool for quant skills development.
Verbal tests are quite good too. But, I found the passages do not reflect the pattern of the actual GMAT. There are only 3 passages instead of 4. And, too many SC questions were left for the end of test.
Overall, these are very good tools for sharpening your skills. But, you must have a mastery of the all available OG questions to be able to understand well the tricks and trades of GMAT. Best of luck!