A couple of nights ago, I was greeted by my two-year old iMac with just a black screen and the Apple logo. In spite of trying numerous keyboard commands, the thing just would not boot. Even a "Genius" at the Apple Store could not fix it, somehow the system had gotten hosed, and would have to be reloaded. Fortunately, I had a recent Time Machine backup and much to my surprise, it worked! All my stuff was there and the computer looked as it had before the crash. Kudo's to Apple for making such an incredible recovery tool, saved me a tremendous amount of pain.