It would be different. To say it would be "much better" ignores that one of the primary usages of Time Machine is that it provides easy whole-disk backup in ca

It would be different. To say it would be "much better" ignores that one of the primary usages of Time Machine is that it provides easy whole-disk backup in case of catastrophe. In this scenario (the primary reason why many people use it), ZFS is completely irrelevant. Now, perhaps one could imagine a future version of Time Machine that would read from source file system snapshots while making backups to the external target in order to capture versions that were made while the user was unplugged from the external disk; and perhaps ZFS would be appropriate for Time Machine target disks in order to save space via block copy-on-write. I'd say I expect both of these features at SOME point in the future ("expect" inasmuch as they're obvious wins and one has to think that the future direction of file systems is to include constant time low cost snapshots via copy-on-write), though I don't know if the file system(s) will actually be ZFS-derived or not. But this is a far cry from saying that Time Machine functionality would be free on top of ZFS. Any future version of Time Machine will include replication to an external disk (or, at least, "the Cloud") and that's NOT a feature of ZFS as it stands right now.