If you have anything else that can use a significant amount of power (e.g. electric stove/hotplate, kettle, gaming rig, bitcoin miner), it's just as efficient a

If you have anything else that can use a significant amount of power (e.g. electric stove/hotplate, kettle, gaming rig, bitcoin miner), it's just as efficient as a space heater. This also means that if your alternative is running a resistive heater, there's no meaningful extra cost to e.g. mining cryptocurrencies with your GPU (assuming you have decent cooling so your components don't get damaged from overheating).