Rayburns (no apostrophre required - it is not the possessive case), Agas and similar products are renown for their relative thermal inefficiency and certainly Aga have been working to try an improve the situation for newer products. The electric ones are the worst, followed by the oil-fired, followed by the gas.
Tamborine has hit the nail on the head - they give out heat to their surroundings even in the summer because they were designed (originally - in the concept) when fuel was cheap was no-one cared.
People who think they are great seem to rely on the heart-tugging issues around 'you can bring a half-dead young lamb back to life in there' or 'the bread you can make at any time of the day smells really good'. Personally I've always thought that lambs were for eating anyway.
EPCs are now bringing these issues to the fore when when considers buying a house - at least you have an idea what you are going to have to fork out for. The price of fuel can only get more expensive relative to average earnings.
You takes your choice.