I have used the Normal Tool in class as an instructional aid while teaching statistics for economists. My experience has been that the Normal Tool is an excellent means of helping students understand key concepts such as the relationship between the standard deviation and the spread of a distribution, what probability distributions are (in a meaningful way), and what it means when a specificr observation falls at a particular point in a distribution. Once my students get the hang of using the tool (as a group, we use a computer lab so that I can help them on an individual basis at first), some students get carried away playing with it and don't want to quit. When they begin to see the connections between the functions of the Normal Tool and the topics we have been covering in class, they have "aha" moments that are really pretty exciting for me. It has made teaching that particular part of my course fun for me and, I believe, more enjoyable and productive for my students.
With instructions, it takes just a couple of minutes for first-timers to become comfortable with using the Normal Tool. Once they get it, they're fine on their own.
I highly recommend using the Normal Tool as a means of helping students obtain a meaningful understanding of probability distributions, standard deviations, population parameters, and so on.