I was disappointed that the water didn't come in very hot, but that's probably because the incoming water temperature was very low. The heater I had installed, an Eemax EX95TC, is a 9.5 Kw unit with a temperature rise of 65 degrees at 1 gallon per minute. The incoming water was probably 45-50 degrees, so the hottest the hot water got was 108 degrees. I was hoping for 115 to 120.
Because the flow rate of the hot water was limited to 1 gpm, the cold water came in 2 or 3 times as fast, so they didn't mix evenly to make warm water. If I wanted to wash in warm water, I would have to set the washer's temperature to Hot, but increase the flow of the hot water to 1.5 gpm.
I'm still learning how to make this work. I expect the heater to work better in Spring and Summer when the incoming water temperature isn't so low.
I did four loads: two large, one medium, and one small. These reduced the water left in the barrels by half, from about 500 gallons to 250. The water flowed from the barrels very smoothly, pumped by a Grundfos MQ 3-35.
Overall the system worked well, and I'm waiting for the next big rain to do another four loads of laundry. This probably wouldn't work for a big family with a lot of children and piles of laundry every day, but it works just fine for me.