Big deal, it'll switch between Sprint and T-Mobile. Those carriers have nearly identical coverage footprints and both rely heavily on roaming agreements (T-Mobile roams on AT&T and Sprint roams on Verizon). There's a very real possibility that Google's MVNO won't allow roaming, and should that be the case, your coverage footprint just got cut almost in half.

Further, neither T-Mobile nor Sprint currently allow MVNOs to use their "capacity" bands for LTE. That means no bands 12 (T-Mobile), 26, or 41 (Sprint). While that may not sound like a big deal, keep in mind that bands 4 (T-Mobile) and 25 (Sprint) - the only LTE bands you'll likely have access to - are on 1,700 and 1,900 MHz respectively. That means building penetration will be atrocious. Additionally, T-Mobile's HSPA+ (3G) network runs on 1,700 and 1,900 MHz as well, and will have equally atrocious building issues. In practical terms, this means your phone will most certainly spend a noticeable amount of time on T-Mobile's EDGE (2G) or Sprint's EV-DO (3G) networks, both of which run slower than cold molasses. Let's also not forget that every carrier, Sprint and T-Mobile included, give lower traffic priority to wholesale (MVNO) subscribers as opposed to branded customers. Enjoy that while at a bar inside a brick building in New York City on a Friday night.