I had very little confidence in bore sighters and bore sighting in general until just recently. Like the other posters have noted, it still took some work at the range to get a rifle hitting where it should; at least that was true of my centerfires.

But a couple of months ago I received a 32" Green Mountain .54 roundball barrel for my T/C Hawken. I also have a Lyman GPR with a 32" .54 roundball barrel that was well sighted in for 70 yards. Both rifles have tang mounted aperture sights.

With time running short until deer season, I saw I could, at most,sneak in one range session before the opener....not good. I have an old Simmons optical boresighter with a set of spuds that I picked up years ago for around $40. I also picked up a muzzleloader/shotgun spud seperately that cost around $15.

Just for the heck of it, I put the boresighter in the sighted-in Lyman GPR, lined up the sights and noted where the front sight fell on the boresighter's grid. I then put the boresighter in the T/C and adjusted the aperture sight (keeping front and rear sights lined up)until the front sight fell on the same place on the grid as it did forthe Lyman. I should add that this point was NOT at the center of the bore sighter grid. I then waited for a chance to get to the range.

When I finally got there I was amazed to find that the T/C was hitting in exactly the same spot as the Lyman. I never had to touch the sights after the boresighter adjustments. And this from a piece of gear that I thought was junk.

If you can do a decent job mounting the scope mechanically, save your money and use it toward a proper optical boresighter and a spud. They appear to have some value in periodically checking whether your zero has shifted.
FWIW
Bob