Better down the drain then to radiate it back into your air. Then you gotta spend more energy pumping that heat outside (if you live in a hotter climate like I do).
Alternatively, use it to warm up the cold water coming into the shower (I'd guess the largest usage of hot water in a home). Warmer "cold" water mixing with the hot water means less hot water used out of the hot water heater. My understanding, placing this only on the cold supply to the shower is it'll only impact your showers. https://ecodrain.com/en/