I have used one in the past.  It is a guide, it's not dead on accurate.  IMO, the main problem with them is that there is not a consistent enough tension on the hay coming past the sensor.  If the hay moisture rises 2% it naturally packs a little tighter and now the sensor thinks it increased by 4 or 5%.  Dew moisture will make them read artificially high.  They will also read differently for alfalf, grass, or straw.  One application I might see for them is if you have hired help running a baler that don't know what dry or wet hay feels like, they can tell you what the monitor says which may be better than "Gee, I don't know, whats tough hay feel like?"