From what I can gather, the overall consensus is that they are more difficult than what the GMAT is likely to throw at you.
As I’ve said in a previous post the gmatclub tests
cover the full breath of the syllabus. It makes it easier to see where you are weakest in.
Bipolarbear – If your scores are going down then perhaps you are not learning from the mistakes you made in the previous tests. You might also be hitting on weak areas in the later tests that you weren’t exposed to in the earlier tests. Perhaps if you can crack these weak areas your average will start to rise again in the second half.
I’m a Computer Science major and have always been good at math but I’m not performing that well on these tests either. I think the key to success is understanding math fundamentals really well – even silly stuff like manipulating radicals, irrational numbers, terminating and non terminating decimals, large factions, exponents… the list goes on.