In the 24 pages of chapter 7 of his book "Making Sense of Evolution", Massimo Pigliucci lays out scientific objections he has with the fairly new (since 1975) field of Evolutionary Psychology. In his book "Nonsense on Stilts", there is a small section where he deals with evo psych, but it is basically just a shorter subset of what is discussed in that chapter.
Massimo has been quoted in a Newsweek article as saying that evolutionary psychology is "not good science". I am assuming that it is in these two books where he makes his case for this. The Newsweek article was an appalling piece of journalism -- intellectually dishonest, and resorting to emotional cheap shots. Massimo's chapter, in comparison, is like a breath of fresh air -- he sticks to scientific arguments and refrains from getting personal.
I am not going to defend everything any evolutionary psychologist ever said. I have not read anything by Tooby and Cosmides, and the criticism I have read of evo psych seems to particularly single out their work. Having only heard the criticism, I don't think much of it, and the things they are quoted as saying fail to strike a chord with me like what I've read from other evo psychs such as David Buss or Steven Pinker.
Generally, I am very leery of things said by evolutionary psychologists that are not backed up with psychological experimentation. It is very easy to do a lot of armchair speculation and call it "evo psych", making up evolutionary stories that are based on things that have not been verified about our ancestral environment, and substituting modern social stereotypes for actual psychological data. But if someone carefully studies what is known about prehistoric life, comes up with theories based on that, and tests those theories with psychological experimentation, that should be taken pretty seriously.
We need to make a distinction here between two different concepts: "Evolutionary Psychology", and "Psychological Evolutionary Biology". In evolutionary psychology, we use knowledge about evolution to cast light on psychology. In psychological evolutionary biology, we use knowledge about psychology to cast light on evolutionary biology. As we shall see, Massimo is mostly debunking psychological evolutionary biology, not evolutionary psychology.
In his book, Massimo is constantly comparing evo psych with the study of the evolution of non-human organisms by evolutionary biologists. It should be noted that for evo psych to be useful to humanity, it has to be at least about as good as mainstream psychology. The study of evolutionary biology of non-human organisms is the wrong standard to which to compare it. If both mainstream psychology and evo psych are less scientific than evolutionary biology of non-human species (as is almost certainly the case), that does not in any way establish that mankind would not benefit greatly from studying evo psych.
One objection Massimo makes is that while evolutionary biologists often study species which have many other species closely related to them, comparison and experimentation between these species being very illuminating, homo sapiens has no close living relatives, the most recent common ancestor of ourselves and the great apes having lived some 6 million years ago. This is obviously totally irrelevant to the issue of any comparison between evo psych and mainstream psychology, since it is a problem both of them face.
Throughout his discussion, Massimo is really obsessed with distinguishing whether observed traits of species are adaptations or caused by other evolutionary forces, such as genetic drift. This is a very central issue to an evolutionary biologist, but it's not very interesting to a psychologist. A psychologist is primarily interested in whether a trait exists, not how it came into being. The difference between evolutionary psychology and psychological evolutionary biology is key here. If a species-wide genetic psychological trait is identified and verified to exist, that is progress, regardless of how the trait emerged.
Massimo says that evo psychs sometimes talk about high level, specific behavioral traits when the evidence may not be sufficient to support such conclusions. That may be true sometimes, but you can accuse any field of intellectual inquiry, including mainstream psychology, with excessive speculation beyond what the evidence would support. This may be a relevant objection to bring up with respect to specific conclusions, but it does not mean that evo psych cannot be done well.
Massimo goes into a lot of detail about the fact that humans are very difficult to study because ethical considerations preclude a lot of experimental methods routinely used on other species. Again, this is a problem shared by both evo psych and mainstream psychology, and it says absolutely nothing about how one is better or worse than the other.
Massimo claims little is known about life during the Pleistocene. He says "little", not "nothing", so it's not clear how much he means by that. I maintain we know enough to make progress, and I deal with this issue in my piece Manufacturing an Absence of Evidence.
Done right, evo psych has a lot to offer. Evolutionary psychologists have at their disposal every technique available to a mainstream psychologist, plus they have the evolutionary perspective, much as an anatomist is better off for being aware of the fact that the organisms being studied were being shaped by evolution, and that most of the structures being observed therefore contributed to survival and / or reproduction in some way. Demanding that psychologists ignore the theory of evolution makes about as much sense as demanding that anatomists ignore it as well.