well i personally use this cutter, all you have to do is refill the blade. although i've had trouble cutting circles that are smaller than 2 inches on it, but i think my blade is dull; time for a new one.
the nice thing about these, is you can cut from the inside of the shape or the outside of the shape.
Plus they come with clear templates to help determine which blade to use to get exactly the size and placement you want.
They're pretty simple to pick up on too-I made a perfect circle on my first try and haven't looked back since!
I have a colluzzle that I love, but is somewhat limited, so I bought another one that they don't sell anymore, only to realize the knife didn't work well...I use my colluzzle knife and its fine! Go figure. The creative memory set is awesome but too expensive for me...its really fun to play with...