In addition to being a whole lot nicer-looking, SpiderOak offers three extremely compelling features its rivals lack: access to your files via the Web, backup of multiple PCs, and long-term retention of previous versions of files. The Web access makes the service suitable for sharing files and folders (albeit without password protection), road work, and saving data from across a small network. The versioning means you can revert to an older copy of, say an article if you somehow managed to edit your way into corner or deleted something important. Not that that's ever happened to me...
SpiderOak lacks the easy drag-and-drop restore via Explorer drive icons that Fabrik and Mozy provide, but it's no big whoop to launch the program to restore files. The view pane from which you launch the restoration also lets you manually remove data from your backup and view changes to files (another feature lacking in Mozy and Fabrik). There are also panes dedicated to sharing and syncing. Yes, the program even allows you to sync directories across the network and machines.
Unfortunately, SpiderOak lacks Mozy and Fabrik's ability to sniff out and add important data files such as Outlook, Thunderbird, or Outlook Express e-mail; Quickbooks databases; Firefox settings; et cetera to your backup. Many users don't know where these vital files are located (they're in hidden Application Data folders) or in many cases realize that they exist. SpiderOak does have a switch that you can turn on in order to see hidden folders or for them to turn up in a search--but it's turned off by default. The program is also a bit of a memory and resource hog.
If it weren't for the lack of help in backing up some major data files, the free version of SpiderOak would be a no-brainer best of breed. It still is for anyone who knows where all their important hidden data is located. Regardless of the user's technical proficiency, SpiderOak is wonderfully useful for backing up and sharing documents and media, with many other nice features I don't have space to cover. Alas, additional storage costs $10 per month per 100GB compared to Mozy and Fabrik's $5 a month for unlimited storage, which makes the latter two still the better deal for those with more than 2GB of data to backup.