I think location has A LOT to do with any issues regarding call quality, connectivity, dropped calls, etc. For example, when I was with AT&T, their service in MN was very good for the most part. When I was in Chicago, it was complete garbage. Dropped calls all the time, failed connectivity, poor voice quality, etc. Same carrier & service, different geographical locations.
I agree with you that poor voice quality reviews, as well as reception issues, likely are the result of the geographical location the devices were tested in. Any problems with the Sprint network won't be the same everywhere, some locations (huge metro areas like Chicago, NY, etc) are much more likely to experience issues then smaller areas with less network resource demand (i.e. users).