We bought a Map Wheel not long after we bought the boat in 1998. Pretty handy little gadget, particularly when figuring out fuel burn for a longer trip (like several weeks) that will incorporate a whole lot of twisting and turning multi-waypoint routes through islands and stuff.
If you use it all the time you will get used to its somewhat unintuitive scale setting procedures. If you use it only a few times a year as we do, don't lose the instructions.