I mention it because I just recently started using it, and today was the first day that it really came in handy. I haven't been using it because I never had a spare drive to use. Instead I had one which I used for a nightly "clone" of my entire hard drive using SuperDuper! (others use CarbonCopyCloner).
When I recently saw a good deal on a 2TB drive, I decided to start using Time Machine with it. Time Machine keeps more extensive backups. Apple describes Time Machine as "hourly backups for the past 24 hours, daily backups for the past month, and weekly backups for everything older than a month" whereas SuperDuper takes a snapshot at a particular time, in my case, 6 a.m. every day. My most important and most frequently changed files are stored in Dropbox which gives me at least a partial "off-site" backups. Dropbox saves every file change for the past 30 days, whereas Time Machine runs once an hour. Time Machine also makes it easier to recover information from iCal or Address Book.
This morning I ran the Safari 5 update on my iMac, and it didn't work. The computer seemed "hung up" and when I determined that nothing was happening, I rebooted it. (That sound you hear is the collective gasp of people who realize that rebooting after a "failed" installation can lead to Very Bad Things.) My iMac would then not let me login. I've since gotten it to work, and the point of this is not to scare you off installing Safari 5 -- this could happen with any software installation. The point is this: with Time Machine plus a clone, I know that I could disconnect the clone drive to prevent it from updating for a few days to make sure that everything is working while still having backups in place via Time Machine. If the iMac had failed to boot entirely, I could have used the SuperDuper clone to boot and restore, sometime Time Machine cannot do on its own.
The moral of the story is one you've heard time and time again: backups, backups, backups. Are you using them? Have you tested them lately to make sure they'd work if you needed them?