It's generally a fine backup against single-drive catastrophic hardware failure, but not against any other failure type: i.e. file system corruption, accidental file deletion/overwriting, destructive malware, etc.
Right - and this is why you take one of the drives out every now and again and replace it with another one! Provided you do that, I don't see why it isn't as immune to the listed problems as any other type of irregularly-made backup.
It might well prove quicker to use an ordinary backup program (my 2 x 2TB RAID1 system takes about 10-15 hours to rebuild, while Acronis will back up the 1.2TB of used data in about 5 hours). And there's a bit of inconvenience, in that you have to shut your computer down and swap the drives. But in exchange, the backup is guaranteed to be atomically that of your computer in its shut-down state - at least on Windows, I think this is difficult to guarantee otherwise.
