I don't see a provision for overflow. I calculated that my 50 gallon rain barrel will be filled with less than 1/4 inch of rain from the single quadrant of roof that drains into it. The rest goes into a hose coming off the side.

I use my rainwater primarily for watering my house plants. Tap water and ground water is extremely hard around here, and the lime buildup will coat the pots white as well as clog the soil and poison the plants. Since in Minnesota we do have to worry about freezing, I have a second rain barrel in the basement I drain it into for house plants in the winter. I keep a submersible aquarium filter in that one to keep the water sweet.