Norton Online Backup is unique in the online backup universe in that you use it and configure it online. There is a local client to interface with your system, but basically the only thing you may do with it is invoke the online interface. This is a bit of a departure from the average service that uses local client software to select files and schedule backups.
The Norton Online Backup online interface is quite nice to look at, and powerful. You can have it choose files automatically, choose files based on rules in additional locations, or simply add them one by one. My primary complaint is that scheduling, while easy to set, is restricted to daily, weekly or monthly settings. This means you can't get the same ad hoc continuous data protection that you can with Fabrik, Mozy and others. Hence, NOB is only suitable for less volatile data sets and off-hour backups.
Norton Online Backup will back up open files, and it did so for me on several occasions, but it did fail once with an Outlook .pst file which was then subsequently backed up successfully. The log stated that the file could not be backed up as "...most likely because some other program was using it at the time." The company is investigating this, but did not have an explanation at the time this review posted. I'll chalk it up to the vagaries of my rather well-used XP installation.
The solo version of Norton Online Backup may be used to back up 5 PCs and up to 5GB of data total. That's more PCs than many services allow, and the price per gigabyte is quite competitive. A NOB account is also bundled free with Symantec's Norton 360 security suite. You get 2GB of storage with the standard version of 360 and the 25GB with the Premier version of 360 as of July 2010.
You may access your backed-up files from anywhere you have web access, part of the joy of the online interface. However, there's no option to duplicate the backup locally so if your Internet is down, so is your access to your backup. However, while online you can e-mail links to files so you can share them with others.
As an adjunct to Norton 360, Norton Online Backup is definitely something you should take advantage of. Even as a standalone it's very competitive both feature and price wise.