Ubuntu One does have some extra features -- for one, more than twice the capacity on the free account (5GB), it can sync contacts, Tomboy notes, stream music (though as a paying option), and also comes as a default on an operating system (Ubuntu).

Actually the only downside for me recently was not being able to share a DVD iso image recently due to the size limit on the free account, and looks like they fixed that problem now too. Contacts sync was a bit of a pain to get working but it works fine now.

But to answer your question partially (from Wikipedia):

> As of April 2011[update], Dropbox has more than 25 million users. [20]

I would guess most are not Linux users.

Also for more serious backup options people should probably use something like tarsnap.

EDIT:

Just found on their blog from February, they claim 2% Linux users. So around 0.5M I guess ...

( http://blog.dropbox.com/?p=339 )

I am curious to how many users of Ubuntu One are on Windows?

Dropbox has always worked well for me and it's cross-platform so I can use it on my laptop also which runs Linux. I like to try Ubuntu One when start to hear more positive things about it and the Windows client comes out of beta.

I'd be more interested in knowing how many Ubuntu installs there have been since Ubuntu One was released to see the uptake rate.