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Last time I checked, Btrfs did not have support for deleting snapshots or subvolumes (which are the same to Btrfs). I haven't been following Btrfs development, so it may now be implemented. It seems like that would be required for this to be very useful...
Also, Btrfs snapshots are per subvolume. Why not have separate subvolumes for /, /boot, and each /home/some_user? This would allow system rollbacks without affecting user data, and would make it possible to allow each user to make snapshots of their own home directory.
Also, does GRUB2 support Btrfs? If it does, next time I install Ubuntu, I'm going use the idea above and maybe write some scripts to implement this with APT.