I have a few cheap 15 buck ones from fleace bay, some uses a 9v batt, the others are smaller and use 3x lr44 batts.
they all read within 1% of each other, really handy when chasing dry wood, for us thats under 10% moisture.
As previously mentioned, to test, you need to check freshly cut, or split wood to get a reasonable reading, rather than just picking up a piece, and putting the prongs in on the outside of the piece, and thinking thats your reading.
Getting readings from the middle of the piece will let you know what its real moisture content is.
usually we just cut, split and stack, and leave it to season for 2-3 years, as the dense aussie hardwood dries out very slowly.
Even dead wood thats been on the ground for 5-10 years will be 30% moisture, as it wicks up moisture from the soil just laying there, or a standing dead tree will still have high moisture content due to the natural capilliary action, and thus, will need at least a year or more to proerly dry out.
So, yeah, they can be handy, and have their place if you need to burn below a certain percentage.
Some may be happy burning at 15% or more, but for us with a wood stove with hydronic back boiler, if we burn wood over 15%, the boiler tubes block up, and if we get a wet piece by accident where water has dripped on it, it can block up the stove real quick.
So annoying on a cold day to put more wood on the fire, only to find the last piece is just smoldering away, and the heat has gone out of the firebox, grrrr.
When its dry, the fire burns with more heat, and its much longer between cleaning/ sweeping the thing out.
so, they can have their place as a useful item on the wood shed shelf.