Hi Saba,
I ended up using PMI's practice exam. Since my work covered all costs for getting my PMP including courses and study materials, I ended up purchasing that exam just to see what it was like. Just be aware that even though they promote it as a simulation, it does not mimic the exam condition. What I mean by that is that the exam is not time based, it is not structured as the real exam, it does not mimic the two breaks, etc. After each question it already gives you the answer and the explanation for it. You would use this more to experience different types of questions from other exam simulators, with a different feel. I still found it valuable as I ended up covering a wide range of question types, from very very vague to less vague. I only used it once, but my main preparation was the PM PrepCast Exam Simulator as this was time based and really simulated the exam environment. This built my stamina to sit down for the final exam and pass with overall AT. Hope this helps.
Regards,