One of the biggest advantages of using an online backup service is that it lets you access your files from anywhere. Most online backup providers let you view and download files from a web browser, but that should be the bare minimum. Many also include file-sharing options, the best of which even let you specify a password for access and an expiration date for the shared item.
Bitcasa starts you off with 10GB for free, but $100/yr throws the floodgates open and gets you unlimited storage for anything you want to sync, share, or back up. Plus, you can connect as many computers or devices to your account as you choose, so you don't pay by the PC. It's client isn't quite as robust as some of the other tools here, but if you're more interested in a flat fee for unlimited space and you can handle the details of which files go where and when, it's a solid option. You can read more about Bitcasa's pricing here.