I don't think there is any "partnership", the "Wifi Assistant" feature just joins open wifi networks. It does do two neat things: first, it establishes a VPN tunnel through a Google controlled endpoint. Second, they seem to have a database tracking the quality of wifi networks. It won't join networks that are known to be slow, and if a network gets too slow it will automatically disconnect.

In practice though, almost every open wifi network is slow. Much slower than LTE, and certainly not something you could reasonably make a voice call over.

Overall I'm relatively happy with Fi. The service is a little better than T-Mobile (still not as good as AT&T) and the management interface is dead simple. Its what a software startup would build if they went into the cellular carrier business. And for my level of usage (3-4GB/month) the price is similar to T-Mobile.

But don't expect it to save you bandwidth.