The topic was online storage and Dropbox certainly offers that. But, it normally offers replication rather than true backup. Replication is similar to backup, but anyone choosing an online storage company should fully understand the difference.
The big difference involves deleted files. Like the previously discussed Mozy, Carbonite and Backblaze, Dropbox deletes their copy of files you have deleted after 30 days. Plus, files in the Dropbox folder can be deleted on any computer connected to a particular Dropbox account. Trend Micro's Safe Sync also deletes the backup copy of a file 30 days after it's been deleted.
If this is what you want or need, fine. But, stuff happens, and files get deleted accidentally all the time.
I often run the Windows Chkdsk utility. More often than not, it turns up errors in the NTFS file system. While it normally corrects the errors, the file system is always a potential point of failure. If it hiccups, files may disappear from the operating sytem without any human involvement.
Replication, also known as synchronization (or sync for short) aims to replicate a folder somewhere else. Programs such as Karen Kenworthy's excellent Replicator, do this with on-site storage. The previously mentioned companies do off-site replication.
With either scheme, files added to the original folder are added to the replicated/backup copy. Files renamed in the original folder, get renamed in the copies. Updated files have new versions of themselves shipped out to the replicated copies.
Deletions are another matter.
When a file is deleted from the original folder, a system doing replication/synchronization will delete the file in the replicated copies it maintains. Granted, the companies mentioned here wait 30 days before deleting files, but they will delete them. A system doing true backup, will not delete the backup copy of a deleted file.
There are exceptions.
Pack-Rat is a Dropbox Pro add-on feature that saves your file history forever. Dropbox saves a history of all deleted and earlier versions of files for 30 days for all Dropbox accounts by default. If you have the Pack-Rat add-on, Dropbox saves those files for as long as you have the Pack-Rat add-on. With Pack-Rat, you never have to worry about losing an old version of a file.
In contrast, a company such as Rsync.net is in the backup businesss. You pay them x dollars for y gigabytes of storage and they never delete anything.
Judging by their name, SugarSync is a synchronization service. How they handle file deletions however, is not clear; even after trying the software, reviewing their website and reading the User Guide. There may be more than one answer as they offer multiple types of folders (Sync folders, Shared folders and a Magic Briefcase). In addition, they maintain a folder of deleted files, and also a web archive folder specifically for files they will never delete (which implies that they normally do deletions).
A final point about Dropbox: their Windows software was not designed with restricted/limited users in mind.
Yes, outside of the corporate world, hardly anyone runs Windows as a limited/restricted user, but I do. It's the Defensive Computing thing to do.
Most software, including Dropbox, will not install when logged on as a Windows XP limited user. Google's Chrome browser is a notable exception.
So, I installed Dropbox while logged on as an administrator and then logged back on as a limited user, and it was gone. There was no trace of the software, it didn't even show up in the list of installed applications (Add or Remove programs in the Control Panel).
To use Dropbox as a limited user under XP (I have not tried it under Windows 7 or Vista), you have to logon as an administrator, change the limited user account to an administrator, logon as the formerly limited user, install the software, logoff, logon as an administrator and change the normally limited user back to limited. You won't find that on their website.