The GM is the one book that if it is right by a hair in any way, then my efforts in ethics will suffer in some way. I've been thinking of the main problem I have in N's GM. My problem amounts to what I take to be an ambiguous relation between nature and culture within his work (at the very least there is a conceptual tension between these two things). It seems very generally that at times the way we are culturally such as being "sick" or men suffering from "bad consciences" is at odds with a more natural way, men with will-to-powers who are stronger, more healthy and do not suffer from cultural forces. Thus, we might say that N is offering us an examination of how we ought to reshape culture in light of how we are naturally. Now, while N might also claim this relationship is an interpretation, it does seem like it is a causal story doing the work for his analysis.
However, there is a real problem I have with this type of thinking found in experimental philosophy. In X-phi, various ethicists are sampling they're 18-20 year old students to see how morality should be structured such that the conceptions of morality respect how it is that we are psychologically constituted. However, the strategies employed are simply polling students with surveys. These surveys occur at a a level of analysis in which I think it is epistemologically impossible to tell where culture and nature can be teased apart. If we can't reliably know the moment they pull apart, then just as it is the case in X-phi, I am unsure how the relationship obtains in N's work such that what is justifying the claim that we should endorse the ways of the master morality over slave moralities (or whatever you take the active skepticism concerning common slave morality in N to be) loses its efficacy. At best, the inability to tell the difference might make us skeptical that N is right into identifying the "correct" side as nature over culture. Perhaps, it is then that nature selects for cooperative behaviors over individualistic ones.
Of course, the immediate rejoinder might be to emphasize N's thinking that culture is just a perverted understanding of nature, and that it is contained within the tent of N's commitment to a type of naturalism. At the moment, I think something like this is probably the case for N's work, though I still think the division in interpretation between culture and nature needs clarification.