They really make a difference

I have been giving GMATClub tests since the last 2 weeks.
To start with, these tests aren't very easy. Having said that, they broadly cover almost all of the sections/topic areas of the GMAT Quant.

One very unique thing about GMAT tests is that they have hidden traps in almost 3 out of every 4 questions that you have to face. This might be discouraging at first, but you should refer to the solution and check what specifically did you miss. This is a very CRUCIAL SKILL for the GMAT Quant.

GMAT Quant sections is not about maths, it's about Quantitative Reasoning. So all you gotta do is Interpret, Visualise and Solve. This means that you need to pay attention to the constraints and all possible cases. GMAT Club Test will get you in the habit of doing the same.

TLDR; GMATClub Tests are the second best Quant Practice tests for GMAT only after GMATPrep.