These trammel heads can accept both a pencil or an included steel pivot point (or a razor knife). The cool thing about this set-up is that they attach to a ruler with the ruler on the flat, so the center of gravity is lower than normal, making the system easier to use. The trammels can also be used in conjunction with a combo square to mark out a line parallel to the board edge.
The heads can be attached to any metal ruler (up to 3/32" thick and less than 2" wide), which means that your standard equipment should be compatible.
As for price, the M.Power seems to be selling for around twenty something dollars, which seems like a good price if this is an item that you see yourself using.