Accuracy of GMATPrep

Based on all the info I have read in the past several weeks the consensus seems to be that the gmatprep tests are the most accurate representation of the test because it uses retired questions from past gmats. I have read some people say it is more difficult and some say it is slightly easier so I guess take it with a grain of salt. More importantly I would use the info to try to find your weaknesses come "g" day becuase in the end it only matters what that score says, but I understand the desire to have a gauge of where you are in te process. Im taking my first gmatprep tomorrow morning after taking 6 mgmat simulated tests so I will probably post something on that experience tomorrow afternoon. Oh and most people will probably mention the "real" gmat has a std deviation of plus/minus 29 pts and that is both true and disquieting due to the relative importance that schools place on the test.