Another very enjoyable Stephenson, if perhaps not his very best. It struck me as somewhat similar to SevenEves [spoilers for that book to follow] in the very distinct 2 part structure taking part in very different worlds. In both cases the earlier, more near-future speculative section was more engrossing, though while I still loved the later full-on heavy scifi second act of 7eves, the equivalent in Fall, for all intents & purposes straight fantasy, never quite gripped as strongly. Also, while 7eves transitioned cleanly between the the two worlds, Fall had a somewhat muddled centre section, before finally finding its feet again in the tale of Prim & the Quest.

None of which to say it wasn't still thoroughly entertaining and thought provoking... difficult to rate though. A true rating might be 5 for the first third, 3 for the middle third and 4* for the final third.