It was time to replace the old sump pump so went with Zoeller given all the positive reviews. I went with the M57 over the M53 because it seemed to be worth the extra few bucks for full cast-iron bottom and impeller. Glad I did as the water is pulled-in from the bottom of the unit (our older sump pump pulled the water-in from a higher level) and our pit is lined with rocks. After installing it I could hear some small stones being pulled-in and ping through the exit pipe, but it seems to have settled-in with no more issues. Once the summer hits and the pit is dry I will line the bottom with some plastic (Zoeller makes an accessory that raises it off the ground which might work as well). Keep in mind the overall footprint of the unit: It is a little larger than our old unit, and luckily *just barely* fit in our small pit. It weighs almost 30lbs and feels very substantial. This unit is 1/3hp and I'd say is about max for the distance I have to pump the water (about 10' up then another 15' horizontal across the house to exit), so be sure to get the appropriately sized pump for your application. Combined with the Cambell Quiet Sump Check Valve our whole system is knock-free and silent - hopefully should last a long time too!