I've been running at home and at work with first-party isolation enabled for a few months now. Google login works fine for me, as does login with Google. It's broken a few internal tools, especially when people do things like hotlink across internal systems (which would have been broken at least some of the time for most people anyway, until they realise that to view _this_ page properly they need to log in over _there_). Also breaks PlayStation Network.
All in all, insufficiently broken for me to be bothered enough to turn it back off :). I do wish there was a "view from alternate origin" feature though, to let me load a site as if it were loaded in an iFrame -- that would let me work around the issues with my internal sites.