Here's Brownfeld's summation:
Shlomo Sand has, in many ways, normalized Jewish history. Instead of the implausible myth of a unique nation with a special destiny — expelled, wandering and finally restored to its “homeland” — he has shown us the history of the Jews as a religious group, incorporating men and women of a variety of backgrounds [as a result of mass conversions], joined together by a common religious belief and commitment, not an ethnic identity. The largely imaginary Jewish past constructed by Zionists beginning in the 19th century, has provoked much conflict and, as he shows, is largely an invention.Where did the idea of Jewish peoplehood come from? Sand traces it to two developments in the mid-nineteenth century: secularization with its eclipse of religious faith and rising German nationalism.
Challenges to the idea that Jews constitute a single ethnic group or people are not new. Rejection of a Jewish peoplehood was at the foundation of Classical Reform Judaism, which held that Judaism is a religious community with a common faith, culture, and set of rituals. Moreover, many have written similarly in the past. As Sand explains:
I encountered scarcely any new findings — almost all such material had previously been uncovered by Zionist and Israeli historiographers. The difference is that some elements had not been given sufficient attention, others were immediately swept under the historiographers’ rug, and still others were "forgotten" because they did not fit the ideological needs of the evolving national identity. What is amazing is that much of the information cited in this book has always been known inside the limited circles of professional research, but invariably got lost en route to the arena of public and educational memory. My task was to organize historical information in a new way, to dust off the old documents and continually reexamine them. The conclusion to which they led me created a radically different narrative from the one I had been taught in my youth.The implications for the Palestine-Israel conflict are profound. To put the matter briefly, on what grounds can the Land of Israel be said to be more mine than that of a Palestinian Arab whose family has lived there for a thousand years?
For details on the controversy see the Wikipedia entry here. Sand responds to critics at his website here.