Really makes a difference!
I'm using one for each tom and two on the floor tom. Didn't need to damp the bass drum but i guess it works the same, only you would need more than two for that. I also use one on the snare.
They cut the sustain that resonates for no reason and that is sometimes bothering when getting through the sound of the other instruments.
Small enough not to hit them if applied on the front face of the drum. Again, they really make a difference.
Very satisfied, I recommend!