Unfortunately it's not fully usable on the OnePlus 6/6T yet. I would love to daily drive my 6T but it only has working audio for the first call before you have to power cycle it. VoLTE doesn't work so you have to use 2G mode if you want to receive calls.
Battery life has been excellent on the oneplus6. As others have said, there are some major issues, such as GPS and the camera not working at all. Calls were giving me issues a few months ago, but nowadays with "edge" (the latest release) they're quite reliable.
With all the bugs and problems, I could use PostmarketOS every day with minimal issues, I used it exclusively for just over a week and it was fine. I think in ~2-3 years I'll be using Linux on mobile as my main device.
Battery life is pretty decent, but I haven't had a 100% success rate with some of the basics like calls and texts. I've enjoyed using it as a kind of mini-tablet though, with no SIM (will keep trying again periodically).