I use Dropbox, Sugarsync, Goggle Drive and Gobbler for on-line backups, file sharing, and file transfers. I've tried several other services as well, but these are the one's that seem to work best for me.
Dropbox is... well, Dropbox. It just works. The restriction of the single root folder is not nearly as annoying as I thought it would be. It works, and it works well. I use it as general purpose storage, to keep certain folders in sync between machines, and for most of my file sharing.
Sugarsync is similar to Dropbox, except you can mirror existing folders and folder trees. That's just so handy. I use it to sync my tech library across several machines.
Google Drive - I've just started with this one, so all I can say for sure is that it looks a LOT like Dropbox, and it works equally well.
Gobbler is just for us computer-musician types. It understands project structures, it can do versioning - although that feature needs some polish, and it works really well.
A couple "fair and balanced" observations:
Yes uploads are slow - but you don't upload large files every day. The initial load may be a bit annoying, or even painful, but then it's done. After that it's all incremental changes, and they do not tie up a lot of resources.
Yes, these companies might go under (well, prolly not Google) - but your hard drive will fail. Seems like a fair bit to me. In the cases of the services above, the data stays on your local computer, only the backup is in the cloud. And that's two of the three dissimilar media... I keep everything on my computers, on external drives, on optical media, and in the cloud. Perhaps I'm a wee bit too pessimistic???