I've been using Time Machine for some time now on my G5 and MBP. With my MBP it allowed me to transfer over my system to a new MBP (pre Lion) in about 30 min which was sweet. This morning I was running several apps. I don't know what happened exactly but my G5 running !0.5 locked up on me. No problem, hold the start button and force restart. Except when the computer restarted I got a ? with a missing System folder, then on a second attempt I got gray screen that would not go away, and finally a apple on a grey screen but the computer would not reboot. Using Restore Your 10.5 System page I worked my way down through the various possibilities, which by the way I added to the MacRumors MacOSX Tips & Troubleshooting Guide this morning. The solution that worked was to pop my System Install DVD into the computer to boot off the DVD, I ran Disk Utility but it told me I could not repair the disk, so on the Main Page, I told it to Restore from Backup (Time Machine). That took about 45 min to erase my startup partition and reinstall my System. At the end, I launched Disk Utility again, repaired the disk, repaired the permissions, selected my startup partition as my system, crossed my fingers, and restarted. The computer started good as new. I believe Time Machine saved me some serious grief. I have no idea why the force restart screwed up my System. It could be at the time I was running an antivirus program when it locked up... hmm. Since since the restore, I've run ClamXav without any issues so I really don't know why. I just know Time Machine saved me.