Mirror HD is a digital mirror that uses the Mac's built-in camera, with no additional options. Despite the application's simplicity of purpose, the icon is pretty much the only thing about it that works well. The image is stretched, extends outside the screen on smaller monitors, and cannot capture images.
There's not much more to say about this particular application. The image it displays is reversed, just like a real mirror. Although it displays a real-time feed of the camera's input, there's a noticeable bit of lag between action and a change in the image (as one would expect of any modern digital device).
I suppose that Mirror HD could serve you well if you really need a fast look at yourself and need not worry about image distortion. The application is fast to load and, since there's barely anything to it, it's a fast download too. If you have Photo Booth installed though, you should certainly forgo this download. Heck, if you have a real mirror on hand that should serve you better still.