I believe horizontals drain up to 300 feet or so around them, that's all with todays technology. I recently read that in Ohio they need to unitize no less than 500 feet from the horizontal. So 300 feet or so seems reasonable. So if in your state they can legally run a horizontal along the property line it appears that you will lose 300 feet or so of gas, or even more if/when technology changes.
As mentioned by Deer Hunter, read your state's rule of capture law.
They cannot drill a lateral under your property, and the setbacks vary from state to state as far as how close they can come to your property line. In states with forced pooling, you could be forced to join a unit and have your gas fracked and taken, but you would be compensated for it. My state of Pennsylvania does not have forced pooling.
Keep in mind, the gas is trapped in the shale which is why it has to be fracked to be released. So it won't flow freely from under your land to your neighbor's who allows drilling.