Personally I feel my bag liner made a significant difference in temperature but the experienced difference may be due to my using a relatively cheap bag. My bag is a featherlite 0° bag and in 37 degree weather I felt cold but with the addition of the liner I did a couple nights of 20° with no more discomfort. I also think the bag liner cut down on draftiness in my bag and kept my skin from directly contacting any zippers. My two cents is that I will happily pack the extra few ounces as I feel I get something in return.