They can be very effective, and even run at a reasonable cost, but only in a well insulated, draughtfree house. Modern houses have high levels on insulation, so it can be a good alternative. In older houses, especially solid brick or stone, they are terribly wasteful. Heat is lost far too soon in the day. When you really need it in the evening, it's often just not there.
The good side is the lower tariff. This varies all over the country, so a bit of research is worthwhile.
With some schemes, an hour's worth of extra low tariff boost is given in the afternoon, before the evening demand.
The heaters themselves have improved dramatically over the last twenty years or so. Modern ones store better, and have much more control over release.
Are these Greater Manchester type "back to backs"?