I've got a Scubalert quacker and a Dive Alert. The latter is for attracting attention on the surface, and the quacker is for communicating with my buddy. Yes, I know you can bang on your tank with a weight, or use one of those little shakers, but I usually have a camera in my hand, so don't want to be carting round an additional weight, etc.
The advantage of the quacker is that not that many people have one, so when I give it one little squirt, my buddy/model knows that it is me calling them. Too many people have tank bangers and you don't know who is attracting who.
I do, on average, 280-300 dives a year, and I have never had any leaks/problems from having the two units back to back under my low-pressure inflator. If I ever did, I'd just disconnect 'em and plug the low-pressure into the inflator directly.
Mark