Amen to that. Had a DM in Fiji last week using some sort of metal rattler constantly trying to corral a group of divers he was watching over and it drove my entire group sharing a site with them insane.The problem with banging on a tank to get your buddy's attention is that everyone within 500 yards, except for your buddy, hears it.
Buddy Link sounds cool but as others have said the price point's too high for the utility you get versus the cost of other solutions. $50-$100 and I bet they'd sell like crazy.