The crystals won't last any longer with the forever tray, it'll just be cheaper to replace because you only have to buy the litter, not the ScoopFree tray system. It also lets you experiment with litters as well. I use the Dr. Elsey's Senior Crystals because my cats don't really like crystals very much. My one REALLY finicky cat only likes clay, but the Senior crystals are much smaller and more clay-like in their feel, so he accepts them. The crystals will yellow with use, but I go in about once a week per litter box and just stir the litter really good so there are not big yellow spots where the kids like to go over and over

. I get about 2-3 weeks out of each litterbox with 2 cats.