I’m surprised Ubuntu One lasted as long as it did. This is what I said about it in my book:
Ubuntu One was designed for the Ubuntu operating system. While the Windows client works now, it’s hard to say how committed Canonical, the company behind Ubuntu, might be to Windows support. Also, Canonical is a company that develops an operating system more than a company that builds file storage and syncing systems. The other services discussed here are more dedicated to the concept of file syncing and storage. It is hard to predict how long any web-based service might last, but the fact that Ubuntu One is not Canonical’s main project is a valid matter of concern for some users.