Tossed a couple that had arrived here over the years as a bonus for purchasing dewormer or something, just last week. Never used 'em, have no intention of using 'em, couldn't tell you how they work. Still in the plastic; I KNOW I didn't order 'em.
IMO, they might be ok if all you fool with are stocker calves, but if you've got a cow-calf herd and you're just going down the alley poking every cow or heifer with the same needle 'til it gets too dull to drive through the skin... well, if you have BLV or anaplasmosis in your herd, you're very likely spreading those agents very rapidly through the herd.