Hi Neil,
I'm really mystified why more people don't have them, especially East Coast snowbirds heading down the ICW in the fall who will be motoring most of the time, anyway... Or, anyone who want's to extend their sailing season a bit, a very worthwhile upgrade, and a lot of bang for the buck...
I have a simple Heater Craft installed in the bottom of my wet/hanging locker a bit forward of my companionway... In a boat as small as mine, it puts out a pretty impressive amount of heat. Even at the end of a cold day of sailing, a modest amount of engine run time going into an anchorage or whatever, will warm things up nicely below... Comes in very handy for drying wet gear, as well...
Only caution, you want to adhere strictly to the installation guidelines, especially regarding running the hoses, they can't be any higher than the engine heat exchanger, if memory serves... Otherwise, the install is pretty simple, biggest challenge on most boats will simply be finding the space within the recommended distance of the engine...
Highly recommended...
best regards,
Jon