Any reason why you want online backup? Are you on the go a lot? What do you plan to back up?
Just reading through the site, they recommend that you upload no more than a "couple of dozen GB" b/c the transfer speeds would be way to slow.
I just use my trusty external and when I'm on the go, I use DropBox.
https://www.getdropbox.com/
Free and provides 2 GB of free storage. You can also share folders between other computers that automatically update as soon as anything is changed. Plus, you can revert back and recover deleted files. However, starting August something, they will only store changes made to files for 30 days but you can upgrade.
I highly recommend DropBox. If you need more storage, you can upgrade (costs more than carbonite) but I have had nothing but great experience with DropBox.