We had one for years on one of our boats... it was getting old and setting up some vibration as the gap where the blades fit had opened up some... In the end we replaced it.
This was not a geared model, so occasionally only one blade would open. This set up a horrible wobble that felt like the strut was going to be torn out. We got into the habit of spinning it briefly in reverse before going forward - seemed to prevent that. The geared types ensure that both blades open together - I believe Martec makes a geared model too.
Slippery while sailing, need to be rotated to a vertical axis to ensure that a blade is not hanging down at slow speeds, and questionable reverse (a problem with most folders, not just Martec) - since we were no longer racing we were quite happy with the 3 blade Campbell prop we replaced it with.
1984 Fast/Nicholson 345 "FastForward"
".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)