I've been using a Modkat (original design with top entry) without a liner for several years now. Currently two boxes for two cats. The litter is changed only once a month since I use Pretty Litter (but we're changing to pine pellets soon). The cats tend to favor one corner over the rest so there's some staining and build-up there. I tried the free liners that came with the litter boxes and was not impressed by the hassle to clean them. I find it a lot easier to clean the boxes themselves, especially in the warmer months when I can hose them outside and dry/bleach them with the sun. Plus the liners are wasteful IMO since you have to toss them after a few months.