They work okay as boresighters to get you close to the centre of paper when sighting in a new scope, about as well as sighting down the barrel of a bolt gun. For levers or others that aren't easy to boresight the old-fashioned way, they aren't bad.
If, like me, you play "musical scopes" a lot, switching scopes back and forth between guns in search of the mythical perfect match, they are great. You look through the scope (previously sighted in), note where the laser appears on the the crosshairs, remove the scope and mount the new one without touching the laser, and then adjust the new scope until the laser dot appears in exactly the same position. In order for this to work, you must insure that both scopes are set to the same magnification, and are at the same height above the bore. You must also use the same target at the same distance (25 yards works well) for both scopes. By doing this you can literally sight the new scope in absolutely perfectly...I've done it many times.
If you are changing scope mounting heights, i.e. higher or lower rings, then this still works if you do it at night, so that you can see the laser dot through the scope at your sight-in distance, 100 or even 200 yards. It is easy to sight your replacement scope in to within 1moa using this technique.
Once again, this is strictly for replacing one already-sighted-in scope with another, and results in the second scope being sighted exactly as the original one was.
