One confusion out there is File Syncing services and Online Backup. These are two different solutions for two vastly different problems. Backups are about protecting your data from disaster. You could have a home break-in, a hurricane, a lost laptop, or a fire and if you don't have a backup of your data, you are going to be VERY upset. You certainly could use Dropbox, but considering the volume of data we keep on our PCs you would have to pay quite a bit to keep multiple copies synced across multiple computers, much of which you probably don't need to have multiples of.
Backups can be as simple as having an external drive that you save your important files to. This will only protect you from a hardware failure. It won't protect your data from theft or from a larger disaster. With backups you you should follow the "3-2-1 Rule". You should have 3 copies of important files (working copy, backup 1 and backup 2) on 2 different types of media (external hard drive, CD, DVD, or online), 1 of which should be offsite (bank deposit box, online).
Online Backup will keep all your data safe and secure in case you have a data loss.
File Syncing Services
Windows Live Skydrive
Online Backup Services
*Some products blur the line between File Syncing services and Online Backup services having both components in one package.
I have used many of the products list but not all. For File Syncing I rely on Google Drive and Dropbox, since they are the only two that support ALL the devices I need access to without strange issues. One feature you'll find on many file syncing services is file sharing. Copy a link to a file and paste it in an email or IM chat window to share the file with someone.
After much research on the topic of backup services I went with Crashplan and have been extremely pleased. What I like about Crashplan is that they provide local backup solution as well as online services. I can even swap disk space with a friend and get online, offsite backups for the cost of a spare drive.
American Society of Media Photographers Backup Overview
Lifehacker: The Best, Most Affordable Alternatives to Mozy for Unlimited Backups
Disaster-Proof Your Data with Online Backup
File Syncing Faceoff: Dropbox vs. Google Drive