I'm really happy this project sounds like it's going well.
It isn't the sort of functionality I envision using myself, at least in the short term, as I'm very happy with git / sftp and the like for the occasions that I need to share files between computers. But this sounds like a solution which is set to be much easier to use, and integrate into appropriate desktop programs (which AFAIK would set it apart from Ubuntu One).
Another nice plugin possibility could be email, for a webmail like interface. For those of us who don't use IMAP or want our email hosted primarily on a server controlled by ourselves (/a server of our choice).