We do add it to our waterers - the chooks seem to like the slightly acidic flavour, and keeping the waterers free from algae is incredibly valuable here. And on very hot days like today (we've just hit 41C in the shade, will test in the sun soon), encouraging chooks to drink is good.
We get the garlic-enhanced version because we do seem susceptible to sniffles and colds due to violent weather changes (it can be 40C one day and 13 the next - it happens - and chooks don't like that) and our anecdotal, personal experience is that it DOES seem to help.
Anything that encourages overall increased health in chooks is good - a healthy chook is less likely to have a problem with a few worms, which is sort of inevitable in free-ranging birds.
It's just not much good against an active worm burden, that's all