The only unique feature I see is WiFi<->mobile handoff, aka fixed/mobile integration.
Apart from that, I get the same service from Google Voice and T-Mobile, except I get supposedly un-capped, un-throttled data (I haven't tried to test this) from T-Mo.
Republic Wireless does the handoff and is slightly cheaper. With Google you buy a Nexus 6 ~$650, RW needs a Moto X ~$300, so I guess it is a cheaper out of pocket cost.
Apart from that, I get the same service from Google Voice and T-Mobile, except I get supposedly un-capped, un-throttled data (I haven't tried to test this) from T-Mo.