For the longest time, Google Now voice search has been superior in capability to Apple's Siri, Microsoft's Cortana, and Amazon's Alexa; however, there is one area where all three of those have Google Now voice search beat. The sound of the voice in Google Now is artificial, whereas Siri, Cortana and Alexa all have a more natural sounding and human-like voice response. Google has apparently changed that.
It looks like Google's latest update to Google Now gives it a vastly improved voice that sounds like an actual person instead of a stilted computer mimic. So far, the only change will be to the U.S. English version. There is no information when/if any of the other languages will get a similar update.
Another thing that is interesting about this "update" is that it must be server-side, because the Google Play Store does NOT include a new update for the app. If you don't have this change already on your Android device, you can manually switch to it by going to Settings > Language and input > Text-to-Speech output > Settings (the gear icon) > Language and choose English (United States) instead of Use system language.
We found a video below that gives a demonstration of both voices as a comparison.