I've seen one in use.... very noisy! seemed to take a long time, but the tree stump was about 18in dia. You would want to have it cut as low as possible before you started.
Just the noise... and the expression on the face of the operator I don't think I would want to do it very often.... leave the stump to rot unless it in the middle of the road. I would not want to clear a pasture that way.
These aren't too bad. Wear eye protection, as is usually smart. Beware of rocks that may ricochet into your shin. This is rare as there are rubber flaps to protect you. Otherwise, like a video game. Try to work all the way down, then inch forward. The tendency is to get too deep and the leading cutting edge grabs the remaining stump. Inch back, go deeper, then forward. You'll get the hang. Check that it is sharp when you rent. If not your cuttings will be finely shredded and you will be all day or week. The cutters will have a radius but the leading edge should be square, not badlly rounded over.