Fair few myths around aircon. The hotels (and shops and government offices) still use it but must keep it at no less than 27C for cooling and no more than 19C for heating. We have ours in the living room on at 26C which is just right. Doesn't get too cold and just a pleasant temperature.
If you get the right type then it is one of the cheapest ways to heat or cool a room. We have a ducted system that cools the living room and bedroom/dining room downstairs and the another that does both bedrooms upstairs. Each is 1.5Kw which gives out up to 4Kw in heat or cooling. It's 19cents per kwh day and 8cents per kwh night. When first switched on one running will cost just under 30 cents an hour day and 12 cents an hour at night.
Once the room is at the right temperature, the inverters slow down using less electric. They do not switch off so don't use the electric for the expensive bit of turning on again, simply run a bit more until it cools down again.
For the heat, you would need 4 x 1kw fires running which would be 76 cents an hour to run in the day instead of the 30 cents max for the aircon.
We also, with the ducted system, have a vent blowing new air in but another vent taking the old air out so no problems with the air getting stale if we have it on at night.
Now, I do love it here in Spain but the last couple of years have been showing ridiculous temperatures with very tropical nights. We've had it more often last year and this year than previous years. Having it on all night only costs about a euro which is well worth it in this heat. Not on much in the day, just when having the evening meal as it's been getting too hot eat outside lately. But although it is cheaper than other forms, even just using 2 euro a day will put 60 euro on your monthly bill. Our normal bill usually a few euro under 50 euro a month but this month's forecast is 96 euro, but boy is it worth it.
Oh, the plus side is the winter months stayed very warm this year and we didn't use it for heating at all.