It works ok. I used it to preheat water but some people don't like the idea of running brew water through a water heater. It can't get over 110ish due to a safety shutoff. I think it can be disabled but I didn't feel comfortable doing it. Was happy to be starting off higher than 60.
The way you adjust temp is by changing the gas flow and water flow. High gas, low water is the hottest but generally caused a shutoff. It worked fine if you play with it so it runs constantly without getting too hot.