They put out the same BTUs per watt as any other electric heater; the laws of physics haven't been suspended. They do sound like a nice heater, but I think they're overpriced.
They do seem to have some advantages, but the sites I've searched make the claim that they don't "take the moisture out of the air" like other types of heating devices.
Well, they don't suck in dry, outside,"makeup" air, like a vented furnace, boiler, heater, or stove, but NO electric heater does. However, ANY type of home heating device raises the air temperature, which lowers the RELATIVE humidity. That is, warm air can hold more moisture than can cool air, so it's going to be drier relative to the amount of moisture it COULD hold, compared to cool air.
It seems that with any new type of heating device, there's always a lot of flim-flam before the government has a chance to step in and stop the bogus claims.
If you vote for government, you have no right to complain about what government does.