Swamp coolers work best where the humidity is very low. My grandparents always had a swamp cooler in El Paso, where the humidity often hit single digits. It worked fairly well there and was inexpensive to run and maintain as compared to regular air conditioning. It also added some much needed humidity to room air. I understand that, in addition to places like El Paso, swamp coolers are common in cities like Denver, Salt Lake City, Albuquerque, Tucson, and Fresno where sufficient water is available.
In highly humid climates, swamp coolers have little thermal comfort benefit beyond the increased ventilation and air movement it provides, which you can manage to get with a simple ceiling fan.
How humid is the area you live in?