We're also happy to confirm that both the Nexus 5X and 6P will work with all major carriers in the US across GSM, CDMA, and LTE, just like the Nexus 6 did. Here are the wireless bands from both phones, per the specs page on the Google Store:
Nexus 6P
- GSM/EDGE: 850/900/1800/1900MHz
- UMTS/WCDMA: B1/2/4/5/8
- CDMA: BC0/1/10
- LTE (FDD): B2/3/4/5/7/12/13/17/25/26/29/30
Nexus 5X
- GSM/EDGE: 850/900/1800/1900MHz
- UMTS/WCDMA: B1/2/4/5/8
- CDMA: BC0/1/10
- LTE (FDD): B1/2/3/4/5/7/12/13/17/20/25/26/29
Those bands cover AT&T, Verizon, T-Mobile, Sprint, and US Cellular, both 3G and LTE, so an unlocked phone from the Google Store will work on all of them with a compatible nano-SIM card. More exotic services like Republic Wireless are not included. International models of the 5X and 6P may vary slightly, but since the odd CDMA/GSM split in the US isn't present in most of the rest of the world, it's not a huge issue. Treat the new Nexus phones like any unlocked GSM-LTE device.
Source: Project Fi