I've largely liked The Official Guide for GMAT® Review, 12th Edition. It's a high-quality, well-built book. A trustworthy book. Makes you feel that when GMAC says that 1+1=10, it's gotta be true. So is the case with mba.com - a well-built and trustworthy website.
Except that I've come across a very small number of questions, the GMAC-provided solutions to which have shook my faith in the infallibility of GMAC's solutions.
Example 1: I was horrified when I saw that GMAC has provided the following essay as a model essay - receiving the highest rating - for an Analysis of an Argument question, considering the following three statements about the Argument essay:
- "In this task, you are not asked to state your opinion but rather to analyze the one given." - Page 759, The Official Guide for GMAT® Review, 12th Edition
- "Perhaps the most serious error one can make on the Argument essay is to disagree with the author's conclusion." - Page 534, Kaplan GMAT 2010-2011 Premier
- "Violate the essay directions, either by not taking a position on an Issue or by taking one in an Argument, and you risk scoring a 2." - Page 539, Kaplan GMAT 2010-2011 Premier
Example 2: Among the GMAT Teaser questions on mba.com, QuestionID=25 succeeds in teasing me. According to GMAC, "The fourth choice is correct. It can be inferred from information given in the third and fourth sentences of the second paragraph: In addition, any institution that holds 20 percent or more . . . the value of the stock would plummet.". I say BS. The passage never says that the stock must plummet. It only indicates that the stock would plummet unless there's an explanation that can be provided.