As for your data (with Dropbox), it's all on your system at the same time it is on the Cloud. I have the 50Gb plan, and one of my slight complaints is that on the same disk, I have the dropbox contents twice (one for my Windows partition, one for Linux) -- so I have to keep 100Gb free total to support them. This is a stupid complaint really (and it MIGHT work if I moved the dropbox to a shared drive, but frankly, I don't want to test out what would happen if two operating systems were using the same dropbox space), so I wouldn't worry about that, but the point is that your files are always your files. If, on the odd chance they do go out of business, all of your files still exist across all of your computers (though they may not be synced anymore), and wouldn't be lost to you.
As for the other aspects of their service, I have to agree that it 'just works'. I don't have to worry about it, ever. I haven't dealt with customer service, so I have no idea how they are, but I've never had to either, and I've been using them since their inception. No lost files, no corrupt files, no strange files, and no problems with version conflicts. It's just... solid.