In my opinion, Opera Unite is doomed to fail.
Do you remember AllPeers project (Firefox extension for P2P file sharing) which debuted about 3 years ago? It is gone by now. Dead. R.I.P.
And there is only one reason for that - it tried to solve a problem which did not exist. Market for this extension was non-existant.
Anyway, as of today, Opera Unite is trying to resuscitate this dead horse again, by offering file sharing functionality in their browser.
But this time, not even with P2P functionality, but with an integrated web-server on your desktop!
Sending one file
Using any decent IM client means I can drag and drop file to conversation window and it is instantly being sent. I do not need Opera (Unite) for this, no need to copy and paste URL and send that URL to other party (which is done probably by IM anyway, so why not sent the file by IM as well?!).
So, Opera Unite does not offer any tangible benefits for one-time file sharing.
In fact, is is much more difficult to use and setup Opera Unite than doing simple drag&drop gesture with your mouse, and probably much less secure (remember, Opera Unite runs webserver on your own desktop computer).
Most people do use IM apps (Windows Live Messenger, Skype, etc.), and they've offered file transfers for ages.
Some of these IM apps (like Microsoft WLM) can even sync complete folders.
Sharing multiple files, syncing changes
As for multiple or repeated file-sharing, using Opera Unite is again too much work and too little functionality.
No sync of changes like excellent file sharing utility called Live Mesh from Microsoft, no option for cloud hosting (so you cannot go offline and are forced to stay online), no automatic file transfers. Not useful for moving files between work and home computer.
Live Mesh gives you freedom, great functionality and ease of use. Opera Unite gives you a webserver. Huh?
When I would like to publish new photos with Opera Unite, my Dad would have to actively download it, and I would have to be online at the same time, running Opera Unite (of course).
With Live Mesh things are much easier - I simply leave files from my camera in my Images folder (being shared by Live Mesh), and Live Mesh magic takes care of the rest. Files are automatically synced to the cloud (this behaviour is optional), I can switch-off my computer, and when my Dad logges-in later on, he will see those images directly in Windows Photo Gallery, on his local hard-drive.
With Opera Unite, there is no cloud advantage (as with Live Mesh) and no changed files sync (as with Live Mesh).
Opera Unite is also not useful for exchanging and syncing documents between me and my customers on large projects (P2P or with a cloud).
Distributing files to multiple users
When distributing files to multiple users, Opera Unite approach is hard to justify. Instead of uploading your files once to a cloud, you have to upload them again and again and again, for each user individually.
In case of Live Mesh or Live SkyDrive, Microsoft takes care of all bandwidth, performance and cost issues (for free).
Frankly, I see many disadvantages of Opera Unite when comparing to existing products, and no advantage at all. Same as with (dead by now) AllPeers extension for Firefox. Sharing file or files with Opera Unite requires more work on both parts, not less, and offers less functionality.
People need to exchange (share) information, work on it, modify it, send it back, and not just passively download few files several times. Background service and optional cloud sync like Live Mesh serves these needs better, one-time file transfer is better served by IM clients (Windows Live Messenger).
Opera Unite tries to solve a problem that does not exist (it might have existed back in 1995, but not in 2009). Same as AllPeers did... R.I.P.
Mozilla Firefox was primarily designed as a browser that should compete with Internet Explorer 6.0. That was not a very difficult task, as IE6 was launched in 2001, and it was really obsolete already in 2004. Firefox ambition thus was not a big one, simply copy IE6 user interface, add Netscape rendering engine, add Google Search box (already known from Google Toolbar), and add tabs (already known for years from Opera).
And Firefox succeeded, now about 10-20% people are using it. We could say Firefox is the most famous copycat. Some advanced users value Firefox for its extensions, but this is a minority, most users install it because it closely resembles IE6 they know, and has some missing features (ActiveX), which make it arguably more secure (but less functional).
Most people installing Firefox never even open a new tab, they use and view the browser as another IE6. You may have noticed that "New tab" button is missing in toolbar at default Firefox installation. They use it simply because it is "in", and see many others doing it as well.
Yes, there are some advanced users, that use Firefox for different purpose, and that is extensions. Extensions are brilliant concept, but they still have many flaws in Firefox. Pro user needs to add 20+ extension to Firefox to make it a bit more powerful then IE6 (not even speaking about superior and fast IE7), and here is where problems start.
Firefox extensions are not tested as a complete package, they come from various independent developers, and can completely break the browser or another extension. They are also huge (potential) security risk, you never know if some extension is not sending out your credit card info.