I am a user of Ubuntu and Ubuntu One.
First: I had a serious problem with ubuntu one. I have a windows and several Ubuntu machine and they all sync one account. My first problem is that i can't really delete files. Whenever I delete something Ubu One resync it, so it appears again in my folder. Furthermore, when i move something to another dir it synconize the new location and resync the old one, so i got doubled my files. I can't reorganize my files. So i tried to delete the duplicated files through the web dash, but i cant reorder and select multiple files by extension. For example i wanted to move all PDF to another location...
Second: I can't configure in the ubuntu one app the loacation of the main One folder. For example i want my One folder to be on another partition than my HOME folder. It is a main problem in linux and windows also. I tried to move that folder in windows with the hardlink method. So i uninstalled the U1, then i created a folder link to another drive, than installed the U1. THEN i lost all my files in my U1. Lucky thing that i have a backup, but i thought U1 is a stable solution for me, and i planned to extend my space, but these bugs are major problems!
It's worth to pay for it if it's working perfectly. I think it's more important than releasing a new version in every month.