With Wednesday's Bcachefs update, the on-disk format has changed, but Overstreet hopes this will be the last breaking change to their disk format.
But with breaking the on-disk format, added is now whole file-system encryption using ChaCha20 and Poly1305, this isn't the fscrypt-based per-file encryption now used by EXT4 and F2FS. Bcachefs encryption is more similar to dm-crypt and should be similar to its performance.
Bcachefs also now has backup superblocks, a new inode format that is more compact and extensible, and many small changes for better supporting multiple storage devices and replication. Kent says that the Bcachefs multi-device support is becoming more robust and usable, including support for caching and tiering. The replication support with RAID is also getting into better shape.
On the user-space side, Bcachefs now has a working fsck implementation. More information on the latest Bcachefs file-system improvements can be found via this announcement. With the on-disk format perhaps being stable now, it will be interesting to see if Overstreet pursues mainline integration in the near future. Those that haven't read about Bcachefs previously can learn more from the project site.