I've found the Craftsman series by Uncle Bob[1] to be very helpful in showing how to do not only TDD, but also pair programming. Before that I would complete tutorials and exercises, but was never able to effectively use it in actual projects. The Craftsman series shows the steps of using TDD in both easy and tricky situations, such as socket code, and enabled me to become much more comfortable with the process.
The visual hierarchical structure is very helpful, both for breaking a project down and understanding it. However, I was worried for a moment that there was no way to have multiple subtasks at the same level, probably because there is no immediate feedback during the dragging. The lack of feedback requires you to guess what's going to happen when you drop the task.
The icons distinguishing guessed from manual estimates are rather meaningless to me. I suppose the manual icon is supposed to represent a set "target"? The guessed estimate could be something with a question mark in it to represent the guess.
As mentioned already, the "Delete" button is where a "Save" should be, I've found myself automatically moving towards it after editing a task.
The tasks could be slightly smaller, so that more fit on a screen (I can see only 6 in a column at most, though my screen is only 1366x768).
Overall it seems like a great tool and I'd be interested in using it. The front page would certainly benefit from a "email when it's beta/ready" box, as my email would be in there now.
[1] http://www.objectmentor.com/resources/publishedArticles.html