When I was cutting lots of circles out of denim and out of cotton (not at the same time) I used the one with the pin that someone else mentioned (is it from Fiskars?). It would go through 2-3 lays of denim, depending how thick the denim was, and I could get about 6 layers of cotton cut. I made one of those fake cathedral ceiling quilts (using two circles, one cotton and one denim and boy was it heavy).
Having said that, I could never have gotten a ring circle out of the one I used, like it shows in the ad. That would be interesting. But my question is, do I really need to get another notion that will probably just sit in a drawer and be used a few times (or never, sometimes in my case) or would I rather buy fabric?