While I will still have to play more to form a better opinion, so far I love the increased zombie count. The game is much more difficult (as if PvP wasn't difficult enough!) and way more terrifying, exactly how I think it should be.
While this is most likely a bug, the reduced zombie count (50-250 zombies as opposed to the 1000+ zombies in 1.5.5) is incredibly unfortunate and makes the game unplayably dull at times. Hopefully this will be fixed soon!
Is not everyone having this issue? I explored about 5 towns in a row and haven't seen more than a couple zombies total. It was totally different a few days ago where there were commonly near a hundred zombies in certain towns.