I want to ask:
- Can someone else see or download my files from my UbuntuOne account?
- Do I have complete privacy or not?
Yes you do. Unless you decide to share those files with another person your files will be safe.
For more information on how to share files publicly please have a look at is there a way to let someone without a UbuntuOne account download files.
And for more information on how a connection to the Ubuntu One servers is done visit the Ubuntu wiki page over Ubuntu One security.