This is what I think:
I took the test twice in a space of 1.5 years. Both times GMAT prep was pretty close. I scored an average of 700-710 during the first time and a 690 on the real one. During the second try, I scored a 740-760 (i took the test 4 times, 4 days in a row before actual test day) and scored a 760 on the real one. So I would say it is pretty close.
Word of caution: If you are someone like me who does not remember the actual questions or the answers after 1-2 days you can use the Powerprep software more number of tries. However, if you are like some of my friends, who remember the question along with the choices, it would be a bad representative of your real skills.
Hope this helps!