One of their big additions was VoIP calling as this meant that calls no longer had to be forwarded to a mobile phone's number. It also meant that calls could be originated over WiFi as well as LTE/4G with pristine quality, and providing a second line without the need for a second SIM card.
My experience with it has been both joyous and a bit of a let down. Calls that come in ring my Hangouts app as much as they pop up in the Chrome browser on my Chromebook. Audio quality is not as good as Dialpad's and when I move from room to room using my Google WiFi network, which is in theory a mesh network, calls drop. When I'm driving, calls drop as I move from cell tower to cell tower, indicating that as the the tower's IP address or the access points MAC address changes the signaling path doesn't know where to send the media to any longer.
On the otherhand, it's a free service so, technical glitches aside, when I'm stationary, and using any of my Android or iOS devices the experience is much like any other VoIP service. The addition of E911 to Work and School accounts is a plus as it seems that Google is taking the steps to make Google Voice a real phone offering vs. simply the "Find Me, Follow Me" service it started out as when it was known as GrandCentral (note I was a founding shareholder and my agency handled the social media for GrandCentral up through acquisition and beyond.)