another vote for Creative Memories circle cutter. The only fault I can find with it is that you can't adjust it to cut many different sizes. But, if you want a good stable cutter - CM is the way to go.
I also have the We R Memory Keepers magnetic mat and circle cutter and while I like how I can cut any size, it's really not user friendly! I end up moving my lines all the time and end up with uneven circles. With CM it's virtually fool proof.