I actually use bcachefs on my main data drive and so far it has been running solid for ~3months. That said, looks like the author has gone camping/moving/re-evaluating life. Hoping the development picks back up or that it gets rebased off of 4.12 (or maybe made into a set of kernel patches?). Wanted a COW fs - tried btrfs and kept running into issues where the drive would fall into ro mode so far not regretting bcachefs.
https://aur.archlinux.org/packages/linux-ck-bcachefs/