I don't, but I know someone that had one and they were impressed with it.
When my tank heater broke down I didn't replace it and just used my stove to heat water when I needed it. Since it was large, I noticed a huge savings in gas bills. The principle of the system you are considering should work similar, saving you money in the long run after initial costs are considered. Like I said I've never used such a system, but I understand that some annual maintenance is required, especially if the water from the mains is of the "hard" type. Didn't sound too complicated to me but if you hire someone to do this yearly that'll add to the cost of it.