A large chunk of my life -- nearly all of my records, accomplishments, photos, music -- is stored on my computer. I got very scared when I thought about losing the data on any one of my computers. Of course, anything super important I store in a few places (like my thesis), but there's a lot of other stuff that I only have stored on one machine -- my photograph collection, my music, some of my research data. Earlier this year, I had the hard drive in my laptop fail suddenly -- I turned my laptop on, and poof, 10 minutes later I hear an awful grinding noise and my computer freezes. Drive was totally dead (it was a Hitachi 7200rpm 2.5" hard drive).
Luckily, I was using Apple's Time Machine, which had an up to date backup... mostly. I forgot that VMWare marks the virtual machines with the no-backup flag, so I lost my virtual machines. I also told Time Machine not to backup my applications to save space on my backup drive, but that had the unfortunate consequence of leaving me with an unusable OS X install after I restored from my backup. But my data was safe!
While I was waiting for a replacement hard drive, I thought to myself, "You know, your backup drive is really really old... what are you going to do if it dies before you can get your data off it?" Not a lot of things would make me cry, but that would probably be one of them.
After that experience, I bought an external hard drive to backup my desktop, but it could fail too. I decided to look at online backup services - for a monthly/yearly fee, I could backup my data and trust (hope?) their data centers would be more reliable than a few external drives. I looked at a few services, but ultimately I decided on CrashPlan. It is a bit pricey, but they have a family plan that lets me backup all my machines, and I figure that $100/year is not that much to pay for the peace of mind of having a backup of my data.
I've been using CrashPlan for about nearly 5 months now, and I don't have any complaints. The service has had good uptime (I had a problem connecting to their online backup servers on Christmas, but it was working the next day), I get good upload speed to them, and when I've tried to restore a single file, it's worked fine. I currently have about 50gb backed up over 3 computers with no issues using a variety of connections (Time Warner Cable, Verizon FiOS, university connection). Overall I'm pretty happy with CrashPlan and intend to keep using it as long as I can afford it.