This would be more useful if you could specify the width and height of sprites and then it would create a grid that you could choose the sprite you’re looking for. Using a cropping interface is pretty much guaranteed to get you picking either the wrong width/height or the wrong x/y.
I haven’t tried it yet, but I appreciate the idea of the tool. Having created plenty of CSS sprites in my day, it tends not to be such a fun task.
A couple of suggestions… one, make an Air version please :) Second, keep a running list of the sections I’ve selected so at the end I have 50 classes that I can just copy to specify all my sprites… third, as vvtim said, allow me to specify a width/height and then replicate that across the entire image in a grid.