The instant demo is a nice touch. Personally, what's stopping me from using an app like this is that I don't trust myself to not absentmindedly shift over to some distraction, or get interrupted by my wife and/or kids while I'm tracking some task.

In those cases, to hold myself accountable, I have to go back to the app, stop the current task, edit the time it stopped at, add a new entry for the distraction, then add yet another new entry to get back to work.