Bottom line up front: Don't bother buying it, it would be a waste. Get it from the library or used for under $5 and you can finish reading it in one sitting. He's basically telling you things that you already know, just in a structured way.
Given that Mearsheimer is a heavy hitter in the IR field, I expected more. The premise of the book is interesting, though the ideas are not nearly thought out enough to warrant an entire book. The "meat" of the book is only about 100 (tiny) pages and within that he repeats himself constantly. It really seems as though he took what should be a 15 page essay and fluffed it up with a bunch of WWII examples he knew off the top of his head and an intro and conclusion that does nothing to add to the analysis of the topic.
Some of his points also don't come across very strong. He outlines a number of cases in which leaders lie, both to their own constituents, but also to other states. That part is logical, though his "proof," which is basically a couple of anecdotes, don't back it up much. However, one of his claims is that states don't often lie to one another--a strong claim at first glance. He admits that he had assumed just the opposite, that leaders lie to other leaders all the time in international politics, but that he was proven wrong during his research. After reading it, I'm still not sold. In fact, he spends a substantial amount of time giving a handful of cases in which leaders DO lie to one another (like Soviet Union exaggerating how many missiles it had, or during treaty negotiation, or in preparation for a war). After reading all of his outlines for this, I'm more convinced that leaders lie to one another all the time! He even at one point says that states that get caught for lying won't have much negative impact once important negotiations come along again, showing there's really no strong justification NOT to lie.
Finally, and this one was really distracting, there were a number of typos throughout the book. They jumped off the pages distracting me from his argument and are peppered throughout. Seriously, Mearsheimer is a respected theorist and academic and this is a professional publication that should have been thoroughly edited. Apparently it was not, and it's embarrassing to see the mistakes. It almost makes me think that all of the people who gave blurbs on the back of the book--including Moises Naim who used to be Editor-in-Chief for Foreign Policy--didn't even give the book a serious read.