We have one gas fireplace in the in-law suite, a regular wood fireplace in our sitting room, and a wood burning stove in our finished basement. Built this house about 5 years ago - have always had a fireplace, won't build a house or buy one without one - but not for heat, rather I just really like a wood fireplace. That being said - our wood fireplace in the sitting room puts out some heat! No insert, in the the sitting room. The wood burning stove in the basement heats up the basement AND we've added vents that allows the heat to really rise up to the main floor. We find them both to be really efficient, although in truth we didn't necessarily design it to work that way! I'm in MD and I doubt we will need to put the heat on until December because I have a fire every evening in the sitting room. That being said - our kids have moved out so it's just the two of us and for the most part our day to day living takes place in the kitchen, sitting room and our bedroom which is directly off of the sitting room. If the boys still lived at home then I'd probably need to have the heat on earlier. Put the gas fireplace in the in-law suite for my Father, from whom I got my love of fireplaces ha ha! But he's older, so it's nice for him if he is up in the colder weather to not have to fuss with firewood.
I do know others who have the insert and swear by them, so I'm sure they are worth the $$$ if you deem it a needed item. I agree with others that if it was included a part of the description of the house you purchased, that it should have been left there. I think it's odd they'd have sold it...and something I would pursue with the listing agent, although if you've already gone thru the final walk thru and to settlement but didn't notice it or bring it up then I don't know if there is much to be done. Good luck, I hope you can resolve it in your favor.
We also have a gas fireplace outside in our backyard, as well as a outdoor wood fireplace built out of pavers down on another part of our property, for a total of 5 different fireplaces/sources. Now that I type that out it does seem a bit excessive!