I have the Tasco Shot Saver with the bore spuds that go from .17 caliber up to .458 caliber and the Leupold Zero Point Magnetic bore sighter. They are good for getting you on paper at 100 yards without having to shoot any shots. Of course this could also be accomplished by bore sighting through the bore while looking at a target.
In my opinion boresighters are most usefull for verifying your zero hasnt changed while hunting. I always check mine after sighting in and write down on paper where my aiming point intersects the grid. After each days hunting I will insert the boresighter and verify my rifle is still sighted in. It is a great boost in confidence while hunting.