I've been remiss in not linking to this review by Walter Russell Mead in Foreign Affairs of The Israel Lobby:

Rarely in professional literature does one encounter such a gap between aspiration and performance as there is in The Israel Lobby. Mearsheimer and Walt fail to define "the lobby" in a clear way. Their accounts of the ways in which it exercises power, as well as their descriptions of the power it wields, are incoherent. Their use of evidence is uneven. At the level of geopolitics, their handling of the complex realities and crosscurrents of the Middle East fails to establish either the incontestable definition of the national interest that their argument requires or the superiority they claim for the policies they propose.

Among many good points Mead makes is that it's not at all clear why M & W themselves aren't part of "the lobby" as they describe it, given that they "describe themselves as pro-Israel, in that they believe in the state's right to exist. They admire its achievements and wish secure and prosperous lives for its citizens. They state categorically that the United States should aid Israel 'if its survival is in danger.'" Mead contends out that "the argument of The Israel Lobby actually seems to boil down to the point that the left wing of the lobby has a better grasp of both the Israeli and the U.S. national interests than the right wing of the lobby does." Or perhaps more precisely, anyone who disagrees with M & W on any specific Israel-related issue is part of "the lobby," while they are just enlightened and objective scholars.

As they say, read the whole thing.