Our solar hot water went live in January 2006, installed by Full Spectrum Solar of Madison. (You may notice from the picture that we have a very small solar window. That is a metal roof, and has nothing to do with the solar thermal.) We have one 4 X 8 Heliodyne flat panel on the roof to provide hot water for 2 people. The heat transfer unit in the basement is a Superstor Contender. We estimate that our unit has cut our therms slightly more than half (from an average of 12 therms a month to 5.5 therms a month). Current natural gas prices are $.88 per therm, so this saves us about $70 a year (until natural gas prices go up). At 11# CO2 per therm, this saves about 60# CO2 a month or 726# a year. That's not very impressive until you add up all the savings from all the units in the country.
Initially, we had trouble getting water that was warmer than lukewarm. The problem was that the temperature on our original hot water heater was set at ~110 and it wasn't a big enough gap to tell the hot water heater to turn on when it was cold outside & the solar thermal water was lukewarm. Once we bumped the thermostat up a little the problem was solved. I am very pleased with our unit, and we have had no problems with it since we solved the initial problem.