I have one of those fin marking kits. we use the circle thingies all the time for marking tubes. they are easier and more accurate than the paper fin templates included with the kits. also they make good drying racks when the fins are gluing on.
the angle thingie is worse than useless: the pencil holder is too small, so it doesn't hold a regular pencil. it doesn't hold a hobby knife either so there isn't a good way to make it work as a tubing cutter. on ours, the angle material (plastic) has warped a bit, so now if you use it to extend the fin lines you will get warped lines up the body tube. the ruler isn't exactly accurate and it doesn't go to the end so it's really useless. the slider thing doesn't clamp real well so it's useless to use as a backstop for marking and it's in the way for most marking you'll do on tubes. anyways the pencil slots don't line up exactly and they don't help get consistent lines, they just hide the tubing and the nice marks you put on the end using the circle thingies. the fin gluing aligner notch doesn't really help much, too easy to screw up the fin alignment in too many ways.
am I cranky? maybe - could be an excellent tool with just a little development. meanwhile, go to the hardware store and get a 2' length of 1/2" or 3/4" angle aluminum for $3.