I visited some other stores today but I only found two units in my town. And both boxes were already being held by customers. It's way too early to put a number on it, but I expect good sales.
Sales are pretty good I guess, but over here (Holland - Randstad) there are lots of units available. I just checked out 5 stores (MM, FRS, Dixons, Intertoys, Bart Smit) and they all had systems. The Intertoys guys sold 3 of the 16 systems they got. MM had ~25 units yesterday, and there were 10 left today.