In general I think these tests are a good tool. Not great. But good. I will give them a hearty recommendation if you have exhausted your OG and Oficial Quantitative Reviews and are looking for alternate materials. Of what I have seen, this is second best.
My complaints are that the questions are often stupid tricky, uninterpretable or unnecessarily complex. In addition, the explanations are suspect at best.
My first concern is that the questions are often unbearably tricky. Sometimes to the point of being unrealistic. They will test your ability to remember that zero is an even number or something ridiculous like that. That being said. You will not make the same mistake twice. Which has to count for something. If you do a couple of these you will immediately know what I mean.
A further complaint is that some of the questions are so grammatically incorrect or poorly written that it is possible to get the question wrong just because you dont understand what is being asked. The one where you have to measure the fish is a good example. The one with the swimmer and the currents is perfect example of a poorly written question because it is possible the current is faster than the swimmer can swim and the swimmer would never reach their destination. Not all the possibilities are considered by the question makers.
I also feel these questions are unnecessarily complex. In the official GMAT questions, fractions seem to have a way of working out. Where as in the challenges it always seems I have to multiply a somewhat unreasonable 23/110 by 7/62 or something of this nature. And it is not that I am doing the question incorrectly, the explanation (which is often lacking at best) tells me I am on the right path. Instead of testing my ability on a concept they test me on multiplication. Blah.
Its almost like whoever wrote these questions took the same concepts that were available and just raised the level of technical difficulty instead of the level of conceptual difficulty. My opinion is that the GMAT is more likely to test your ability to understand and integrate concepts rather than your ability to multiple and divide by obnoxious numbers.
In all, I would estimate that about 20% of the questions are really good, strong, thoughtful questions that are representative of what you would find on the GMAT Prep software. The other 80% fall into a category such as grammatically unsound/not interpretable, trick questions or unnecessarily technical.
That being said, I would still recommend these. They do teach you to look for tricks. They do force you to preform long division and multiplication of fractions by hand. And those 20% of good questions are challenging. Most importantly, they time you and put some pressure on. That all adds up to a worthwhile expenditure as long as you have exhausted your official materials first.
On a side note, is it possible that these percentile rankings are no longer valid? Is it possible that the same person has written the exam multiple times, done really well on their subsequent attempts and is therefore skewing all the results downward?
If I took my average over the first 8 of these that I have completed it would be somewhere around the 50th percentile. Which is a Q36ish. When I took the GMAT Prep 1 exam prior to the GMATclub challenges
I scored a Q48 which is like 85th percentile. I will be interested to see what my Q score is on GMATPrep2 and the actual exam to see if these challenges do indeed have a positive impact and do actually reflect my abilities.