Well, first of all, I keep the bathroom door open which helps a lot. Even then, air circulation is so so only. When they are in the middle of pooping or peeing, you can definitely smell it. But after about 5-10 minutes, I barely smell anything. I have to stick my nose closer to the bowl (or in the bowl) to smell anything.

To me, it smells way better than when using cat litter. I used to use the normal clay type, which was great for the first few days, but gag-inducing after that. Switched to flushable arm&hammer, which was better, but it has a sweet smell that can be annoying. I keep the bathroom door open, and guests never notice any smell when they come in, but they did smell something when I used the litter box in the washing area next to the kitchen.

The only exception... if your cat has soft poo and some sticks to the side of the bowl on the way down. So far, with the kiddie seat in place, all poo and pee go straight into the water. From my room, i can hear the plinking and plonking of my cats using the toilet.