GmatClub Diagnostic Test

I took the diagnostic and I felt like it tested a lot of finer details. But the GMAT Club tests, in general, are really good practice material if you want to hit the 700+ level consistently. You might score low on these tests, but it'll be fruitful in the end to look at concepts you've missed and brush up on them.

As far as the diagnostic goes specifically, I'd say that you might want to just look through the answers and explanations and gauge your weak areas roughly and then use some other guide book like MGMAT or Quant Review or so to brush up on ALL the concepts before getting back to GMAT Club tests.

I might be wrong, but I feel that GMAT Club Tests should be towards the latter part of your preparation.

Hope this helps!