This sticky black ooze is one of the best things to help your cats with hairballs. We have two cats, one of whom has a very weak stomach. But since we've been using this hairball remedy, we've seen a decrease in the number of hairballs he has.
It's a very thick black paste that consists mostly of petroleum. The flavor of it must be good, since we can just put some on our fingers and the cats will lick it off. You can also apply it to the cat's paw, but this stuff tends to stain, so I prefer the finger licking method.
The vet has also recommended to us if the cats eat something they're not supposed to, give them this. It helps things pass through their digestive systems quicker.
Overall, it's effective at what it's supposed to do - eliminate the hairball problem.