There are lots of variables here.
Electric heat is the most inefficient means of heating. While a resistive heater dumps 100% of the electric energy into the air, you're paying for all the downstream losses. A reverse AC cycle is a bit better since they can have higher coefficients of performance. The kind you're looking at is purely resistive.
I wouldn't be using it for any practical amount of space. Our cousins have it in a bathroom, and it's nice there since they're in a slab and the tile floor can get cold. It's practical there because getting out of the shower means warm feet on cold tile.
But that's avery intermittent use and fairly small area. IMO it's too expensive to use otherwise. If you could install hydronic in floor radiant Id consider otherwise, since you could use the floor area as a giant radiator and heat with more cost effective natural gas. But even then Id think it carefully, since you're wasting a lot of energy heating the slab/dirt below it.
I think good slippers would be my selection. Ugg ones are pretty nice.