I am backing up all of my photos and docs, so I have purchased 160GB of storage. Yes, it is a lot of photos...
I am finding that my system is running very slowly while the backups are taking place, and Ubuntu One is taking up a massive amount of my CPU time. System monitor is reporting it using 40-170% of the CPU time. I also can't easily view my 'Pictures' folder, i seems to hang for a long time figuring out which folders have/have not been backed up yet (I guess).
I am running Ubuntu 12.04 LTS with all latest patches. I have laptop with a Intel i5 processor with 8GB ram.