Here are my primary uses of Ubuntu One:
1) Sync files that I might want to load from another computer of my mobile phone. I do this with my ebooks and FBReader on Android.
2) Sync Tomboy notes, which I can view from any computer or my mobile phone using Tomdroid. Having your notes available anytime, anywhere, is extremely useful.
3) Sync photos taken on my mobile phone to my laptop automatically. Syncing videos takes a little manual work, but I'm told that's being resolved.