Why does my computer (quad core, 4gb ram) slow to a grinding halt when ubuntuone is syncing!!! I'm on 11.04 and getting really fed up with it... I can't do anything when it syncs!
If this is the initial sync and you have many thousands of files, there is a known bug in Ubuntu One which causes high CPU usage. It should, after a while, drop to normal levels.
However, if you stop it before it's finished, it will start again.
A good page for issues with Ubuntu One is the Bugs Wiki which answers most problems.