Yes, you absolutely should do this.
There are two basic types, one type is all the way into the foreplace cavity, flush to the outer fireplace surface. These would likely be more expensive to rip things out and install, and they absolutely have to have the fan going to give you heat. I dont like that. Although, a freind has one, and she does get some heat by leaving the glass door open during power outages, it is not enoughheat realy, and you would want to realy have some sort of backup power at least for the fan.
The other type is more like having a wood stove and you are sending the stove pipe up your fire place flue. Wether this is DIY or not depends on the person. There are varying degrees as to how much these stick out from the original fireplace. If it was me, and I had the room, I would want it to stick out, if not all the way, at least a fair amount, even if that meant I needed to extend the hearth area to accomodate. Given my feeling on fans and heat, I would actually be very inclined to just get a regular wood stove, totally all in the room in front of the old fireplace and just use the fireplace to run the woodstve pipe. But, a good compromise is to have an insert that at least sticks out enough that there is a surface to set a pot down on ( so maybe halfway out ? ) and that way, even though it will usually use a fan to get all the heat, you will also get convection off the metal if the fan isnt working.
You should state here what you situation is. Is this for primary heat ? Or just back up heat ? How much room is there by the fireplace etc...
I live in an area where, due to fuel prices being crazy, no-one in this area uses their gas or electric heat any longer, and wood stoves are primary heat. And we have power outages. Your situation may be different