Dropbox is a quality alternative to Ubuntu One which will be disappearing soon. Dropbox works well in Ubuntu and is easy to set up by downloading a DEB file and installing it. The integration and file syncing across computers and devices is perfect. Dropbox ONLY offers 2.5GB for free and charges $99 USD per year for 100GB of space. I wont go into too much detail here since most computer users know about Dropbox, but it is one of the top options instead of Ubuntu One.
Dropbox is what I currently use (as well as Ubuntu One), but I am looking at other options that work just as good. Look for another article soon covering other alternatives to Ubuntu One. :)