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The GMAT has just a very special and elegant way of asking questions, which most unofficial questions fail to imitate.
Echo this. That's not to say that every single third party question is bad. But, on the whole, you're better off sticking to Official GMAT questions. When might you use third party material?
1. Skill builders. You may want to do a whole bunch of tough exponents practice but not find enough of it in the Official Materials. In this case third-party questions can be helpful.
2. There are some questions that are under-represented in the Official GMAT Materials that I know are on the real test. For those questions I'll supplement using Atlantic GMAT or other companies' materials.
3. LSAT Logical reasoning for GMAT Critical Reasoning practice.
But, at this point, because there are a whole bunch of Official GMAT CATs (and you can use QP1 to make little exams + don't forget about GMAT Focus) I don't assign any third party CATs to my GMAT tutoring students. Why bother if the scoring and the flavor is off? The exception: if someone has been studying for a year and is completely out of materials. In that case, you don't have much choice. Hope this is helpful!
Happy Studies,
A.