My experience, for what it's worth:
I have a Webasto Air top 2000, as the main means of heating a 24 foot cruiser which I have lived on for 4 years.
The heater normally runs 3 full days and 4 evenings per week in the winter, using red diesel.
On purchasing the boat, the heater was not working and I took it to BK for servicing, they cleaned out the soot and replaced the burner unit. After working for about a month, the heater again failed due to clogging up with soot. Another trip to BK and they diagnosed a loose fan on the shaft and replaced the fan unit as well as another new burner unit.
Since then the heater has required a new burner unit every 12 months or so, (1000 hours ish?) which I have replaced myself. (Note that these are cheaper if purchased through a HGV specialist rather than a marina, I use AB Butt in Leicester)
I made two modifications in December 2006:
Fitted a fuel filter - I was expecting the burner to need replacing about December 2007 as usual, but still going strong, so an improvement there.
Removed one of the fuses - the installer had fitted 2 x 10A fuses in series for some reason. This was causing an unacceptable voltage drop on starting with a lowish battery, as mentioned in another post. I can now start the heater on a frosty morning without having to run my engine.
In general I am happy with the heater now, but would be interested in other peoples experiences.
M