The methodology has been questioned, as have some of the researchers' assumptions and their inability to foresee some "unforeseeable" factors ( eg. air quality).
What is often overlooked in the population question is the strong opposing side that argues that population, far from being a depleting force on humans as a whole, becomes a motivating force to continually arrive at new solutions, balancing the population's growth. Additionally, even now most geographers do agree that there is sufficient food currently available to feed the existing population of the world, and that there has been for some time. The problem exists, not in the resource itself, but in the ability to distribute the resource to the areas of need.
Is population a concern for the coming century? Will technological advances continue to balance growth? If overpopulation is a concern, what are acceptable measures to limit population growth?