I got the Gmatclub tests free with my e-gmat course. It came with 25 odd Quant CATs and 9 Verbal CATs. I have taken a few of both and I feel that the quant is top notch. The difficulty level is a bit above the actual GMAT but that's not really an issue as it just helps you understand concepts more thoroughly. The DS questions too encompass pretty much all the tricks that one can potentially be tested on.
However, in my opinion the verbal is a bit far off from the actual GMAT. The quality of questions isn't that great and on a number of instances I was not convinced of the answer choice even after going through the explanation. On the contrary, in OG or Gmatprep software, the answers and the reasoning are always clear and convincing.
In summary, I would say that Gmatclub tests are as good as they come for QA, but they are better to avoid for VA.