I've used this a few times to clone a failing system over to a new one without causing downtime, insecurely copying data or wasting time block-copying the entire disk - simply add the target disk to another Unix machine and run the following:
sudo env SSH_AUTH_SOCK=$SSH_AUTH_SOCK rsync -a --delete --progress -x --rsync-path="/usr/bin/sudo /usr/bin/rsync" / USERNAME@CLONE_HOST:/PATH_TO_CLONE_DISK/

This approach has the advantage of being potentially very fast - I've used it on "semi-production" systems (e.g. important enough to care about downtime but not enough to have some sort of redundant fail-over setup) by running rsync to copy the data, shutting down key services and doing a mount -o remount,ro on the volume in question, rerunning rsync and then halting the old system while the new one is booting. Scripted this approach can result in only a few seconds of downtime.