I clean mine once a week, and the only smell is "animal" smell. It's really not that bad, though some people may find it offensive. (It's kind of like people with dogs or cats... if you aren't used to the "animal" smell, you can walk into the house and know right away that person has dogs or cats. Likewise if you ARE used to that smell, you can walk into the house of someone who doesn't, and notice there's a different smell, that lack of an "animal" smell.)

I recently read somewhere on this forum that added a few drops of vanilla extract to the water supply can help cut the odor out of the urine. I don't know if it actually works or not, but I'm going to try it out to see!