I want to tell you something about my experience with the my.pen-circular black / red here.
In the 9th grade, we just have radiation rates, and since this has something to do with standards of figures or the magnification factor, needed a new circle after my Rotring CentrO kicked the bucket after many years of loyalty.
So I thought: I'll take the right model for my pen; and what shall I say? It was not just a pure fit-for-my-pen-and-is-usually-not-impressive-buying, but a truly sensible purchase. I look forward to a long successful years and circles with this well-made and existing not only plastic circle. Exactly suits my needs, which were also covered by the Rotring.
Also helpful is the "sandpaper-level", on his circle one gets after long hours again Maths pointed easily.
Well, and that's really the main points on a compass, should mention that it can draw circles. :)
Strong buy recommendation!