For my computers with Natty UbuntuOne only works more or less stable with the nightlies releases ( https://launchpad.net/~ubuntuone/+archive/nightlies ). My main concern is, when I add the repo to my apt sources I get prompted with a warning that says that the key is not trustworthy. Now, how serious is this? I'm syncing tons of personal data - so I wonder whether I can trust nightlies or whether I should not use it? Any idea? How big is the chance that my data are being read by a third party?
Any nightly repository is likely to come with some risk of breakage (code wise), but as for your data - no one is reading it if you are using the same service as what you would use with the stock Ubuntu One.
The reason you are getting an error in Apt is because you did not add the gpg key for the repository (which I suggest you do, but it's not a huge deal).