I used to use DropBox when I was uni to sync all my essays and lecture notes across all my PC's. You get 2GB for free, which at the time was good.
Since then, Microsoft has released Windows Mesh. This offers you unlimited file syncing between specific computers (which have to be online at the same time to sync) or up to 5GB on Windows SkyDrive. It seems like a better deal and it's built directly into the existing Windows Live framework.
My opinion on cloud-based backups is that it is safer from a direct and intentional hacker. Data centres will have spent a lot of money on firewalls and other forms of protection. This makes it more difficult for someone to get in. On the other hand, there will also be more people trying to gain unauthorised access due to the commercial nature of cloud-based backup solutions.
I think in most cases, you have to just acknowledge that NO data is 100% safe. You can make it harder for someone to get to it, but in the end, if someone wants it that bad, they will get it.
I've been using Acronis Online Backup and it's garbage. Doesn't work more than half the time. I just updated from ATI Home 2010 to 2011 hoping to fix it and Online Backup does not work at all now. I'm going to request a refund.
I'll be moving to Carbonite in the near future as I've heard nothing but good about it.