For people who aren't heavy data users, this could be an amazing thing. For those of us who do use network data extensively... not so much.
The availability of unlimited data programs is tough to beat, and I can understand where Google doesn't want to go there (at least initially). As it stands, I'm paying $35 after taxes for unlimited talk/text/4g w/ 3GB 4g hotspot (T-mobile loyalty discounts for long-time customers rock!), so the value proposition just isn't there w/ Google as a direct line replacement (especially when I average 12-20GB/month of mobile network data).
That said... it's still something I'd consider as a swap-out SIM supplement for travel, though, as stacking Sprint's network availability on top of that of T-Mobile would cover a lot of dead stretches of road (and vice-versa for Sprint users). With no contract, if there's no activation fee associated it would be great peace-of-mind to have it as a failsafe for areas where T-Mo's network might be absent or spotty (not that I've personally found many spots that fit that description).