Smells weird and could be overwhelming
My kittens definitely notice when I put it in their litter box, although it doesn't do great job. Better than nothing, but put exactly the amount that's written on the label otherwise you risk your cats to turn their backs on the litter boxes until you put "normal" litter again. It's not expensive so I'd recommend having one on the side in addition to other ways of fighting the smell (closed litter box, good ventilation in the room, odor neutralizing cat litter, etc.)