I'm using it on Gnome (I've tried really hard, but I can't take to the Unity interface). My son is set up as a plain old garden variety desktop user. Nothing complicated. And to be honest there aren't many settings to fiddle with! I limited him to 1 hour every day, between 1500 and 1700 every day; there didn't seem to be anything else to do. The Timekeepr tray notifier tells him there's so many minutes/seconds left, and then nothing happens! But if I log out or switch users, Tom's account is then locked and I can't reaccess it.
I hope this isn't a bug that shows up in Natty. Timekeepr was the answer to all my problems!