When it comes to data other than music, the Ubuntu One Music app is no help. But, you can still access the rest of the data by logging in to Ubuntu One through the browser. On my iPhone, I was able to log in to Ubuntu One and navigate through the folders I have synced, and open a DOCX file stored there. Within whatever viewer or filter Ubuntu One used, the file didn't display properly--it seemed like a blank screen with the doc title at the top. But, by tapping "Open in Pages" I can open the file in the Apple iWork Pages app, or by tapping "Open In..." I can select another app--like Documents To Go--to open the file.
Ubuntu One also syncs contacts, notes, bookmarks, and photo syncing is listed as an upcoming feature. There is an app called Ubuntu One Contacts available for the iPhone, and I wanted to use that to sync my contacts from the iPhone to Ubuntu One, but that app is not available...at least, not in the United States.
One advantage that Ubuntu One has over iCloud is the inclusion of Android. You can keep files, music, and other data in sync between Ubuntu Linux, Windows, iOS, and Android. That may be a valuable distinction for users who are not iOS-centric and have an Android smartphone and an iPad, or an iPhone with a Motorola Xoom. Of course, that comes at the expense of the Mac OS X platform, so you will have to assess which platforms and devices you use and determine which cloud syncing service meets your needs.
Some of the features are in beta, or are a work in progress, but the foundation is there, and it seems to be quite capable. The base package offers 2GB of online storage for free. Additional storage can be added 20GB at a time for $2.99 per month, or $29.99 per year. The streaming music service is an additional $3.99 per month, or $39.99 per year.
In contrast, iCloud provides 5GB of storage for free and doesn't charge extra for the music features--so a user with 4GB of data to sync who wants to stream music will end up paying $70 a year more for those services with Ubuntu One than with iCloud. iCloud also seems more polished, seamlessly integrated, and with broader capabilities than Ubuntu One (like syncing apps and ebooks), but iCloud won't be available until fall, and it will be iOS-centric. Ubuntu One is available today and works with Linux and Android.