After years of fighting with an unreliable diesel heater, we are very pleased with the Espar heater we installed in 2000. It is one of the bigger ones they make and keeps us toasty in all weather.
One thing we would suggest, though, is not to use the exhaust exit in the hull that they normally specify. In heavy weather this will be very vulnerable to flooding. We had Mike Bowden at Ocean Options, where we bought the heater, fabricate us a 4′ high chimney with a ‘Charlie Nobel’ type top that is braced to the stern rail. With this setup we can use the heater even in very heavy weather at sea—when a wick type heater would almost certainly blow back—which is, after all, when you want to be toasty below. The drawbacks of this type of heater are the substantial electrical use and complexity, but these are balanced by the convenience of just flicking a switch when we want heat, even at sea.