I have been using the GMAT Club tests for the past 2 weeks now, thought i'll write up a short review before my subscription expires. So overall, my experience with the tests has been amazing. The quality of questions is the main stand out factor i believe. The questions are tougher than the official GMAT questions, but not "unsolvable within the designated time" tough. They are based on core concepts and test your depth of knowledge. The analysis pane is also an amazing feature to review your performance. The solutions are accompanied by forum discussions, making the solutions clearer. The UI is amazing and has the same layout as the GMAT. Have not faced any technical issues.
On the downside though, there are no complete tests. You can only give sectionals. I think that we should have the option of giving the verbal and QA sections together with an optional break in the middle. This will help simulate exam conditions.
Also, I am looking forward to more verbal questions. There is a heavy imbalance between the number of questions for QA and Verbal.
Overall, in my opinion GMAT Club Tests is among the top 3 tests available for the GMAT.
And they will only get better with time.