I honestly didn't think she'd use it at all until much, much down the road, but she LOVES her circles! She collects the pieces, sorts them by color, counts each stack, and then asks me to stack them. We talk about what whole, half, third and fourth mean. She loves this activity and seriously does it with me nearly every day we play downstairs.
It was also ridiculously simple -- I just cut four circles out of thin craft foam, then eyeballed halves, thirds and fourths, and labeled them appropriately. I demonstrated once how the circle worked, and she has just run with it.