tell me about fireplace inserts

[mattkime]

Sam3 wrote:
We are considering getting a pellet stove insert for our lower level fireplace that can't be used due to the flue having cracks and being degraded. Though they are not very attractive, they do generate heat very inexpensively, we are looking to lower our gas bill.

What kind of gas? Natural gas? I'm surprised pellets would be cheaper than natural gas.