We had hurricane Isabel come through here last fall. I spent a solid 2 months doing nothing but cutting up fallen trees. You could not buy a stump grinder for a million $. There simply was not one available within a 1000 miles of here.
After seeing everybody in a rush/panic to get all their stumps ground up, I feel like I've seen every mistake that could be made with a stump grinder.
Now that the initial rush to buy them is over, they are finally becoming available again.
Get a BIG one. Take the diameter of the tree stump and ask what size grinder you need, then get the next bigger size than what they recommend.
MAKE SURE it is hydraulically powered for up and down and the back and forth motions. MAKE SURE the grinding bit itself is powered hydraulically as well. You DO NOT want a chain, belt or gear drive.
Make sure the grinding teeth are sharp or fairly new.
Grind AT LEAST 6" below the surface of the ground, better to go 1' below surface.
If it's pine you're grinding and you want to grow grass where the stumps were, remove the ground up stump debris and replace it with some top soil. Pine acidifies the soil and grass does not like acidic soil.
Do it yourself, unless you're a real office jockey.
If the machine drives itself and everything on it is hydraulic, it's a piece of cake.