What features does Ubuntu One provide that others services do not have? Can I use Ubuntu One only on Ubuntu machines? What about sharing data/folders with others who do not have Ubuntu One?
Ubuntu One has several advantages over Dropbox and other services.
Like other services, Ubuntu One allows synchronisation of files across machines and does this by synchronising any file you put in the
~/Ubuntu One folder. In addition, it allows you to synchronise all of the files in any folder in your home folder. However, it lacks the ability to synchronise files using symlinks.
A particular feature that Ubuntu One has that is lacking in other services is the synchronisation of your desktopcouch databases. Some applications use these databases to store data and settings. This allows automatic synchronisation between applications on multiple machines. Examples of such applications include Tomboy (which synchronises its notes) and Evolution (which synchronises contacts).
Storage Amount and Pricing
Ubuntu One provides a relatively generous 5GB of free storage compared to 2GB provided by Dropbox and SpiderOak and 1GB provided by Wuala. With Ubuntu One you can add 20GB increments for 2.99 USD/month. This allows flexible pricing that is more cost effective than Dropbox or Wuala. (See Comparison).
The Ubuntu One client is available on Ubuntu and Ubuntu based distributions and should theoretically work on most other distributions. A beta is available for Windows. There are also apps available for Android and iOS.
There is also a rudimentary web interface for accessing and publishing files. The publishing feature allows you to share files with people who don't have Ubuntu One.
This is not as many platforms as, for example, Dropbox so this may be a disadvantage if you use multiple platforms.
Ubuntu One provides a music store and allows music streaming via smartphone applications. The music streaming costs money. Other services do not support this.
The Ubuntu One client is free software, although the server is proprietary.
One advantage of using Ubuntu One is that eventually many of your settings will be saved in a CouchDB database and shared between all your machines (should you choose to allow it). You can do this manually using Dropbox or similar services, but with Ubuntu One, it should be more or less automatic.
One disadvantage of Ubuntu One is that you are stuck with a directory "Ubuntu One" (spaces and capitals, ugh) in your home directory that you can't remove. According to the Ubuntu bug tracker, the solution is to remove U1 completely.
Dropbox allow you to put their "Dropbox" directory anywhere. I put it $HOME/.db/Dropbox then have a symbolic link $HOME/dropbox to it.
As for security, Dropbox does encrypt the files, but they possess the keys and reserve the right to allow the U.S state to see what you are holding. Since they possess the keys it means that hackers could also get them. Ubuntu One does no encryption. However it is possible to encrypt data on your own machine prior to syncing it with Ubuntu One and it will be safe since you alone would possess the key. There is another service called Wuala that does encryption, while making sure that they have no knowledge of the encryption key. They store the data in several European countries in a peer-to-peer thingy for redundancy, they offer a Linux client and the servers are run on 100% green energy!
Canonical closed down Ubuntu One cloud file services 2014: http://blog.canonical.com/2014/04/02/shutting-down-ubuntu-one-file-services/