Hi Laura, if you make a lot of circles, I recommend the olfa circle cutter! It works GREAT:
Cut perfect circles from 1-7/8 to 8-1/2 inches in diameter.
Features built in blade cover for safety and a plastic guard to cover the pivot spike when not in use.
Great for fabric; paper; vinyl; film; leather or more.
For right- and left-handed users. Includes RB18mm blade.
You can get it cheaper on ebay, amazon or joanns with a coupon.
http://www.amazon.com/dp/B001CEAMCY
It makes perfect circles adjustible to 8.5inches. I was doing a lot of Peekaboo or mock cathedral window quilts as you go and it cuts circles thru several layers at once. I had 60 c ircles in a blink. It uses standard (small) rotary blades--easy to change. I wish I knew how to post a photo. Haven't mastered that yet. Hope that helps.