Haithan:
I cannot speak towards the Lehman exams, but I can tell you that I believed the PMP Prepcast Exam simulator exams were a bit more difficult than the PMI exam and were very close in term of wording and "trickiness" if that is a thing.
Personally, I did 4 complete exams off the simulator then went though the every knowledge area/domain until I did almost all questions in the pool. I passed on my first attempt, 5xAT.
Hope that helps,
Harry