I bought a Mercator to replace one I had as a kid. The plain carbon steel has to be in the RC53-56 range. It sharpens about as easy as any knife I've ever had, and I was actually able to hear the burr snap off when I was sharpening it. I brought it up to #8000 grit and can shave with it. It doesn't have much wear resistance compared to the steels I'm used to, but it's fun, and does what it's supposed to.
If you want to teach someone how to freehand sharpen this and the Douk Douk would be my recommendation due to the steel, blade shape, and cost.
I recall from when I was a kid they will rust, but mine has been great, no problems with just a wipe off.
It's a good steel to practice things like mirror polishing, convexing etc too.
The lock actually works ok as well.