Have farmed with them since 1948 when we got our fist WD. Great tractors and no special problems that we had. As has been mentioned the older high use ones may stick in gear or jump out of gear. Not a difficult fix. Rain will enter around the shifter boot if it is cracked or worn and get in the tranny oil. In winter this will freeze and can lock it up or bust things.
The manuals are all available and I would highly recomend buying a full set. Available through this web site I believe. Norms may have them.