You can do client-side encryption with Dropbox or Google Drive, nothing is stopping you from doing it.
I have an encrypted EncFS volume that I keep in Google Drive and it has been working really well, being synced between 3 computers. I'm on Ubuntu and in Ubuntu it's easy to create an Encfs volume with the Gnome Encfs Manager. Just make sure you pick a strong encryption password.
What I like about this approach is that I don't want all files to be encrypted, only my documents, certificates, licenses and so on. But I don't want to encrypt the PDFs in my technical books collection, because I want those PDFs to be accessible from anywhere. I also have an iPad for example, I use the GDrive client to download the PDFs and open it in Adobe's Reader.
Therefore I have two problems with alternatives to well established cloud storage, like Dropbox and GDrive:
- hosting your files on S3 or on some instance somewhere costs more
- I need clients for Linux (workstation), OS X (workstation), Windows (my wife), Android (my phone) and iPad (my ebook reader)