The only one I have experience of was in a charter boat which I used to keep an eye on for the owner. The only comment that I would make is to make sure that you have a dealer with good access to spares when required. On the west coast of Scotland, getting spares was difficult. Apart from that, the only problem we experienced was as a result of "the bug" in the fuel tank. It gunged up the Mikuni a treat!
I used to have an Eberspacher which I have to say gave me no end of trouble and was frighteningly expensive to repair so I researched diesel heaters and in particular the Mikuni.
Knowing that the Eberspacher was just a lorry heater that someone had put stainless into to make it less likely to corrode it was refreshing to see that the Mikuni was a boat heater designed from scratch to be a boat heater.
It seems that the Mikuni is full of new innovation that the Japanese are so good at bringing to old technologies and is thus reliable and efficient. I decided in the end not to fit a heater but if I ever change my mind I will definately opt for the Mikuni.
A friend has it on his motor cruiser. Works and sounds just like Eberspacher, though his has been a bit problematical, needing the unit sending back for repairs
I bought an '89 boat with a Mikuni '30' installed. Works great, but current draw is high on start up - as they all are.
I have sailed all year round and often put it on whilst motoring up estuaries etc. to warm the cabin through before retiring below.
A word of caution. My Fulmar has an 18hp Volvo 2002. I once thought it was knackered because it wouln't rev. The cause was a cosy night at anchor having used the Mikuni extensively. The alternator was taking more hp at low revs than the engine could muster, and that was without any exotic charging aids.