Ubuntu One Basic renamed Ubuntu One Free and storage space increased to 5GB
voices.canonical.com
voices.canonical.com
Unfortunately I think Canonical has garnered a bit of a reputation for pushing alpha- and beta-quality software as stable to unwitting end users--PulseAudio, Unity (and the latest Compiz), etc.--and Ubuntu One was no exception. I would never, ever trust that kind of release mentality with my mission-critical files. I'm sticking with Dropbox all the way. Though congrats to their reaching 1 million despite that!
I am not sure how the Ubuntu LTS release being stable has anything to do with their Ubuntu One service...
zOS, perhaps. Solaris, HP-UX and AIX too, but don't expect having the latest version of Ruby on them.
Lucky me it didn't work, because it was impossible to sync my local files, they were "too many". The One client spend days "indexing" my files and never managed to get that done.
Apparently that is or was some bug in One. If they sell 50GB packs they maybe should offer a client that can manage some 100K files.
After some days of trying and hunting bugs I deleted One from my PC and have no intention of trying it again.
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 ...
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.