Services like this have some of the same characteristics of a system with RAID disks. You get a N+1 copy of each file that you designate, but not what I'd fully call a "backup".
The problem is that if you either corrupt your file(s) or delete them, that change gets dutifully corrupted/deleted on every machine, yielding no good copies anywhere. So, if your machine gets hosed in the right way, you *will* have a good copy in the cloud. However, if you hose it in the wrong way (like with a nice recursive delete that starts in the wrong place), you're still hosed. If someone steals your machine and deletes the contents of your Dropbox, the stuff is apparently gone for good unless you're lucky enough to have one of your other machines *off* the network at the time.
There's an indication on Dropbox's website that their premium Business service has version-control, so it's possible that it doesn't suffer from these types of non-failure failure.
The lesson is -- don't keep your only copy of a file in your Dropbox folder, because it's not "backed-up" against cranial failures. You still need some other type of backup solution to really be protected.
I felt in love with cloud storage a couple of years ago and I've happily avoided tons of problems since then. This kind of services are really handy if you work with several different platforms as I do too.
As a fast alternative, you can use a Microsoft or Google email account to get a web browser enabled cloud data storage service -- I discovered some months ago that my old and largely unused Hotmail account provides me some GB of free storage now ;-)
But only some days ago, my smile turned into a sad face when I received a mail in which I was informed about Ubuntu One data storage is going to be closed in a few weeks...
@Brisco66: Copy.com, managed by Barracuda, is another cloud storage service that works very similarly to Dropbox, except that you start with 15GB of storage and get an additional 5GB for every referral...
Very interesting -- I'll look into them. Thus far I've been extremely happy with my DropBox account -- but maybe I'll open an Copy.com account and then use that for the bulk data like huge images and suchlink while keeping DropBox for my smaller, mission-critical work data...
Copy.com, managed by Barracuda, is another cloud storage service that works very similarly to Dropbox, except that you start with 15GB of storage and get an additional 5GB for every referral (compared with 2GB and .5GB for Dropbox). Shared space is also divided among shares on a pro-rated basis, so 1GB shared is spread betweeen the number of people sharing.
If you join from a referral, you also get the additional 5GB, so here's a link to Copy.com. If you use this referral, you'll start with 20GB.
BitTorrent Sync is an interesting peer-to-peer alternative to DropBox. No middle man. Google "btsync" for more info. [I'm not connected with them in any way, but I've successfully used the service for over a year].