I think it's not earth-shattering, but still very useful. Those who read WPW probably already know how to share large files with reasonable security, and have server space to do it. Unite would be good, however, when you want to transfer a large file without storing it on a server, or good if you're not privy to server space or otherwise restricted. I personally do not view it as a public server or public file-sharing solution. My own use of it would be restricted to more private transfers between who I choose or even just myself at different computers.
I also think that Operas' killer feature is the voice/speak add-on and Unite doesn't trump that for me.