It's actually pretty good stuff. I've been putting in Bob's food (dry) for a few months now and his breath is much improved, his teeth are a lot cleaner too.
I haven't used it so much on my cat Ebby's food though, I think mostly because she seems to notice it on her dry food and is reluctant to eat. Her teeth are pretty good anyway.

On a vet tech's recommendation Randi got some for her cat Fister a few months back too. Not sure how well it's worked for him, forgot to ask for an update.

I can't remember the price offhand but, depending on the size of dog, there is enough for around 6 months and works out pretty economical.