On Sunday morning I got up and couldn't get my MacBook Pro's screen saver to stop. I had a spinning beach ball instead.
Force-quit didn't deliver any relief so I did a hard reset.
When it restarted, it got as far as the Apple logo, then crashed. I tried three more times and the same result. I tried booting from Tech Tools pro DVD I have but it was a few months too old to boot the new MacBook Pro.
Next, I booted from my Snow Leopard DVD. Ran Disk Utilities and repaired the disk. Or tried. The first repair attempt quite halfway through and the second quit right away. The drive name disappeared, replaced my a default machine name.
I had to reformat the hard drive even to install Snow Leopard.
This would have been a disaster, but I'd been using Time Machine to back my laptop up to a 2TB external hard drive. Except for having to drive to work to get the drive, it was amazingly easily.
When the OS X install finished, an option screen came up asking me if I wanted to transfer data from another mac or restore from a time machine backup. I plugged in the USB drive and two and a half hours later, I was up and running and I'd lost a total of one day's work (time machine incrementally backups once an hour when I'm plugged in at work).
That's pretty great- one day of lost work and that's it.
Time Machine rocks.