These kinds of UI tweaks can be amazingly frustrating to users if you're not careful. For instance: When I click on an empty input box, I expect the cursor to load on the left-hand side of the box. In the date box in the demo, it populates the mask and places my cursor in the exact part of the mask that I clicked (usually toward the middle). I have to click again (on a much smaller target area) to move the cursor to the beginning of the input box.

I filled out a 3-page form yesterday with this exact problem. It was memorably annoying and had a significant negative impact on my opinion of the company behind the form (ING direct).