I owe my Q50 to gmatclub tests!

Pros:
1. Best quality 700+ level questions. These questions give you a hint of where exactly GMAT plays a ploy on you. Once you do enough of these, you kind of get the possible ploys that could be played on you for most types of questions.
2. Only high quality questions from the forum are picked up for the tests. Link to forum discussion and Bunuel's solution to each question makes you wonder how can such a seemingly tough problem be so simple!
3. Prepares you for a tougher game than real GMAT, making real GMAT quant much smoother than you expected.

Cons:
1. Do not trust the scoring if you are not at Q48+ level. You get a few 500 level questions wrong and the system will classify you as a Q35. Just focus on what you got wrong and mark it in the error log. That brings me to the next point.
2. Question bank is heavily biased towards 700 level questions. This is good in a way, since it is aimed at those who're looking to score a Q51, but screws up scoring. I get two consecutive 500 level questions wrong and I'll be greeted with a 700 level question next.
3. Quality of questions for verbal needs to improve significantly. The difficulty level is judged by the time stats and for many of these questions, the reason the time stats look so odd is because the question is of poor quality with not so justifiable OA. Many of these questions are still being debated in the forum.

While I wouldn't recommend this for verbal, for quant this is the only material where I could find for tough, high quality questions.
On quant again, don't give too much of a darn about the scoring, but focus on the error log from these practice questions and over time with a GMAT prep mock, you will see that you've become better.