We have this one:
Barbas Unilux-3 40. We've got through nearly three times the wood we did last year with an open fire (although it has been a long winter), but it kicks out five times the heat, so we turned the living room radiators off which saves a bit of expensive (and non-renewable) gas. The heat spreads about the house as well, so the boiler is on for less time now saving more gas.
If you don't run them hot enough, you'll get corrosive creosote forming in the flu which will eventually eat it's way through, so getting a fire of the correct output for the room it's going in is desirable.
However... If you're running a back boiler, you'll need a fire that's bigger than what the size of the room it's in dictates (if that makes sense). With the correct output fire for your room, you'll not get enough heat in the room while it's heating the water, and with a fire that's big enough to do both together, if you're not heating the water the room will be like a furnace.
