A space heater is a highly efficient device as it converts almost all the electricity it consumes into heat. Nevertheless, it can be considered eco-friendly or energy-efficient only when used to cut down on the energy consumption of a central heating system.
In other words, the cost-effectiveness of a fireplace insert largely depends on its usage. For example, during fall, if your intention is to eliminate the chill in the room you're occupying, you can certainly opt for the usage of a fireplace insert and save some expenses. But, during winter, if your goal is to keep yourself warm, you'd be better off switching on your furnace due to the electrical needs.
If you're looking to calculate the cost of using a fireplace insert, please visit this article.