What's the advantage of using Ubuntu One vs. other services like Dropbox?

Ubuntu One has several advantages over Dropbox and other services.

Synchronisation

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).

Platform Support

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.

Music

Ubuntu One provides a music store and allows music streaming via smartphone applications. The music streaming costs money. Other services do not support this.

Freedom

The Ubuntu One client is free software, although the server is proprietary.

The paid Ubuntu One service helps subsidize Canonical, which means that (theoretically) signing up with U1 helps improve the platform.

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.