I'll second SvenW's warning about silent failures; if anything, it's a little too good at surviving a drive failure. I've seen the aftermath of a couple of servers that had one drive drop out of a software mirror for some reason (I suspect not coming ready in time after a reboot); everything works fine off the remaining drive until, several months later, something goes wrong with THAT drive -- and it switches back to the drive that glitched the first time, and the last few months have vanished.
Here's a short shell script I whipped up to fix this. Substitute in your email address, save it as something like /etc/periodic/daily/150.check-raid, make it executable, and it should mail you a warning (at 3:15 the next morning) if the raid ever degrades. To test it (strongly recommended in case of spam blocks, etc), plug in a couple of disposable drives (USB keychain drives, whatever), mirror them, unplug one, leave the other overnight and see if you have a warning in your mailbox in the morning.
# This script checks for any degraded/offline/failed/whatever software
# RAIDs, and if any are found emails a note to an admin. To use it,
# replace the ADMIN_EMAIL value with your own email address, drop it in
# /etc/periodic/daily, and change the owner to root. This'll make it
# run its check every morning at 3:15am.
# Warning: this script doesn't check anything other than software RAIDs
# built with the Apple (i.e. Disk Utility) RAID tools. It does not check
# any hardware RAIDs (including Apple's RAID card), or even any third-party
# software RAIDs. If "diskutil listraid" doesn't list it, it's not going
# to be checked.
if diskutil listraid | grep "^Status:" | grep -qv "Online$"; then
diskutil listraid | mail -s 'RAID problem detected' "$ADMIN_EMAIL"