We recently installed a stove and had to consider whether to go for a back boiler or not. Most of the advice we received seemed to be based on the idea that the fire is on anyway, so you get free water heating if you have a back boiler. This is not correct.
The fact is that stoves with back boilers yield less heat output to the room as some energy is being used to heat the water in the boiler. Or to look at it another way, if you want the same heat delivered to the room, a boiler stove will require more fuel than a non boiler stove.
Also, if you just use a boiler stove to heat domestic how water in your cylinder, you can sometimes overheat the water which ends up in the heat being dumped via the expansion pipe into the header tank which does nothing for efficiency.
We decided that the extra cost and complexity of the plumbing, the extra cost of the boiler stove, the extra ongoing fuel costs etc did not justify any benefit that might be gained and we opted for a non boiler stove.
Since we installed the stove, I have kept a careful eye on the running costs and can definitely say the following:
1. The stove is definitely more efficient than an open fire even when it is not in use as it reduces drafts.
2. The stove is definitely less efficient than our central heating system (a bog standard kerosene boiler and rads etc). (Therefore a good decision not to go for a boiler stove.)
3. Because of point 2 above, the stove will never pay for itself.
4. In spite of point 3 above, we love the stove and would recommend it.
Hope this helps you decide.
3CC