My request for using Ubuntu-One Beta was approved by Canonical, so I immediately made a small (& un-scientific) test. I put 13 files, totaling 23,4MB, first in Ubuntu-One and then in Dropbox.
It took 5' 56" for Ubuntu-One and 4'25" for Dropbox in my ADSL line to synchronize the files. For the time being, Dropbox is faster. Furthermore it has the significant advantage of being cross-platform, while Ubuntu-One works only in Ubuntu. I use Ubuntu both at home and work, but I also use Fedora and rarely Windows and moreover I want to share files with people that don't use Ubuntu. So in my case I can only use it as personal cloud storage space and not for sharing. What's the point of having files in the cloud if you can't access them from every available platform? (I would also like to see both in Solaris and BSD, as well)
Ubuntu-One's speed will probably get better in the future, but unless Canonical makes it cross platform, I really don't see anyone (including me) to keep using it.