The 4 year old hard drive in my intel iMac was failing. I replaced it using tools and instructions from I fixit.
The repair went very well. I had my Mac fully restored and running within 2 hours from the start of the repair.
I back up my hard drive regularly with Time Machine. With os x lion, all I had to do was boot the new drive from a recovery USB drive and erase the new hard drive. It was easy and fast.