But those backup methods are nowhere near enough. I use one of my external hard disks for regular clones of my entire startup volume, again using Personal Backup. I run this backup every few days, and I run it manually before any major OS X update. (I don't want this to run automatically, in case something I installed causes problems, and I only discover the problems after my clone has been overwritten with the new software.) This doesn't take long, as the backups are incremental—they only copy new or changed files.
If I have problems with my startup drive, or with Mac OS X, I can instantly boot from that volume and try and solve things. Even though I don't run these clones daily, I have my other backups of user files on another partition on this backup drive, and can access my Time Machine backups at any time. I needed this a few months ago, when, suddenly, my Mac froze, and I couldn't reboot. All the data seemed fine, but the startup volume was no longer "blessed," meaning that Mac OS X didn't see it as a valid system disk. I booted off the clone, copied over my Users folder, then cloned the clone to the original startup volume and rebooted. This too an hour or so, but much less than a full restoration with Time Machine would have taken.
Backups in the cloud
I'm not a big believer in "the cloud" for backups, because of the time it takes to send data to a server. My outgoing bandwidth is only 100Kps, so large backups take hours. Nevertheless, since I have a MobileMe account, I use Apple's Backup to run an incremental backup of my home folder to MobileMe once a week. I've never needed to recover any files from it, but who knows? I may need it some day.
Better safe than sorry
Just in case you thought the above was obsessive enough, there's one more link in my backup chain. While a lot of people suggest off-site backups, it's not really practical for me: I work out of my home, and I can't store a backup, say, at my office. So I have a fireproof safe in my basement, in which I store backup disks.
I have two sets of backup disks, which I rotate every Friday. One is a backup of my media drive, and the other of my startup drive. I use hard drive mechanisms in a disk dock, so the disks are easy to transport up and down the two flights of stairs. Since I have two sets of these disks, one stays in the safe, and the other stays on my desk.