Considering H1Z1's ropey start and its intent to eventually be free-to-play, the fact that it's sold one million Early Access versions of the game is good going.
We have sold over 1M copies of H1Z1 on Steam! — John Smedley (@j_smedley) March 24, 2015
I haven't played for a few weeks admittedly so I've not seen some of the newer features yet (the zombie hordes sound interesting) but the current state of the game is good; mainly because of the murderously unforgiving nature of most of the other people playing. Hordes aside, zombies are the least of your worries. The real terror is hearing the footsteps of another player in the building you're looting and knowing only one of you is leaving.