I think google would be the one company I would trust with this. I wouldn't put anything too personal on there but I would trust them not to lose what I do put on (or shut down a month later leaving me to find another solution.). I would probably use it to store my firefox bookmarks and my thunderbird account data and emails, etc. This way every time I re-format I don't have to back that stuff up.
I wouldn't trust google with my hard drive..
Especially with the recent cases where companies and governments have been taking them to court to get user information.
Google could certainly find out a lot about the behaviors of computer users besides online, and I'm sure they would create products to benefit financially from it. It's a smart idea, but I won't use it. Hackers is what worries me.