Exam Pack 1 from GMAC has different questions from the basic GMATPrep (free) version. I found Exams 3 and 4 to be very helpful when studying and would highly recommend taking them as the last exams leading up to your official GMAT. Beware that some questions may repeat in Exams 3 and 4 (I ran into that issue with higher difficulty verbal questions).
I would also highly recommend Question Pack 1 from GMAC. It has about 400 official questions that you can sort based on difficulty.
The GMAC resources are definitely worth the investment. In my opinion, the official resources from GMAC are by far the best material to use when studying. If you can master and truly understand these questions, you will be ready for the actual exam.
Best of luck!