How big are your cats? I see up to 15lbs listed, but I've read a few people with 20lb cats that use it just fine. My cat is 20lbs, 5lbs over his normal weight, but he got fat after recovering from fatty liver disease. He loves closed in spaces though, so I figure if he fits, he'll use it just fine. I'm just a little weary of spending almost $400 and have him not fit.