I bring this up to defend Opera Unite from what I see as unfair charges that it’s bad at file sharing, because I think it’s not designed for file sharing.
What is it? An incredibly potent extension to the toolkit of web apps, allowing them to do hacks which were fundamentally impossible in the past. For example, they could do interframe communications using RESTful interactions, by bouncing them off a local server. And they can receive asynchronous notifications! That’s a big deal.
Of course Opera Unite is sucky at doing the kinds of things that traditional web servers and file sharing tools excel at: it’s running inside a web page. Nothing you or the Opera people could do to fortify it would change the basic deal that it’s a daemon in a highly unstable operating environment. Uploading big files doesn’t make a lot of sense if the page the server is in is likely to disappear at any moment. On the other hand, uploading little things like RSS files and CVS diffs can pack a big punch.