mysterymanrog, is right - you get a decent degree of inflation - you save a ton of time (unless you solve through them) but again, you would know the answer. It is fine to retake by the way - you still get good experience and perhaps a bit less traumatic one since you know some of the questions but the scores are not reliable. Sometimes even a handful of questions can inflate your score.
Interesting enough, GMAC published results of a study a long time ago (need to dig on their site to see if I can dig it out claiming that knowing an answer to 1-2 questions did not inflate/change one’s score due to adaptive nature of the test). Not sure how accurate that was but they said 1-2, and not 3 or more. They were addressing cheating concerns and things like that.