I installed a Keystoker KA-6, dual fueled unit for a customer the other day, and he is loving it. It puts out 200 gph of hot water, heats his house like there is no tomorrow, and does it for very little money. In the summer time, expect to burn 250 pounds of coal every 2-3 weeks for domestic hot water only, and in the winter time about 50 to 100 pounds a day for heat and hot water, depending on how cold it is outside, how large the house is, and also how well insulated it is.
The nice thing about the dual fuel is if you go away for a while, or if you get hurt and can't do the coal. It takes about two minutes to convert it over to oil and vice versa.