I have multiple sleeping bag liners, including a silk liner for early season scouting/ fishing/ hunting trips. I love the liners for multiple reasons; most importantly keeping my expensive sleeping bags cleaner. After a few days in the field I am usually pretty nasty- the liner might postpone a wash for my bag for another trip. I totally agree with 25contender, that sticky feeling of oily skin on nylon is gross. Every time you stuff or wash a down bag, down is being damaged and loses its loft; i.e, our expensive bags are dying. The liners also trap some body heat, boosting the "temperature rating" of the bag. You can get away with a bit lighter bag using a liner, but saving your bag from the dreaded washing machine makes it a smart choice to add to your kit.