Some issues.
1) How much. Given their small size, I bet they're fairly expensive.
2) Is your data encrypted, or can their staff read everything you send to them?
3) What do they store their backups on?
There are many other, well-established hosting sources.
http://www.strongspace.com
http://mozy.com/
http://www.jungledisk.com/
I use Strongspace myself all the time. But personally I'd go with Jungledisk if I was in your situation.
You have to think about their system's uptime. If a website you like isn't available when you want to use it, it may not be the end of the world. But if your backup solution has issues just before your hard drives decides to kill itself, then you face losing a lot.
JungleDisk is just a little app that sits on top of Amazon's S3 service. It's incredibly reliable, since it uses all of Amazon's know-how of keeping their servers available. And your data is spread across lots of disks, meaning that you're unlikely to have your data lost by them.
Amazon S3 is also very cheap to use (even just to try out), JungleDisk is free, and your data can be encrypted for more security (also my recommendation).
Keep doing your other backups too though. Don't have total reliance on them.