It's just a larger, lovelier, portable version of the trackpad on Macbook Pros.
Let me re-phrase that - it's just a larger, lovelier, portable version of the trackpad on Macbook Pros that will up the productivity of a billion people.
I've used a Magic Trackpad at my desktop for a couple of days now. The killer gesture is the drag. Yes, it drags windows around, which is handy, but it moves anything around, such as image boundaries in 'crop' mode, and window corners to re-size the window, files or other objects from hither to thither.
Oh, oh, oh! I've just discovered the 3-finger backward-forward gesture. My life will never be the same.