My 6P is not my daily driver, but I installed the O beta about 5 hours ago, and the only strange behavior that I've seen is that the phone seems to be treating my Project Fi SIM as a T-Mobile only SIM. Even restarting the phone hasn't changed that. I may have to try clearing the cache and data from the Fi app to see if I get a response.
EDIT: I take it all back. I powered off and back on, and the Fi SIM went through it's usual activation steps. So now, the 6P is just working like any other phone. I just wish its WiFi setup was as good as my Nexus 6's (but weak WiFi reception was true well before any version of O was installed on this phone).