> If the fsck is not able to restore the data and gives you crap instead, then it's useless
But, IIUC, this is exactly what existing fsck's do. If they can recover data, great, but their main priority is to make the filesystem data structures internally consistent again. If that means randomly making up data that's missing, throwing away some files whose metadata got confused, etc., then oh well, too bad.
Hopefully btrfsck will have a mode where it uses the checksumming information to give you a guaranteed-accurate list of which files were left in an inconsistent state with potentially screwed up contents, but if so then that will be a fancy unique feature that has never been seen in a mainstream Linux fsck before. (And does btrfs even keep data checksums by default?)
I can understand why Chris doesn't want to release a known-buggy fsck, but let's be realistic about what a bug-free fsck actually does...