I have one that I only used a few times. Never noticed an increase in warmth and it's a bit of a tangly pain. I suppose, keeping your bag clean would be a long term benefit, but I find sleeping in a merino base layer does that without the irritations a liner comes with.
Any more, if I have to use a liner, it's an Integral Designs Vapour Barrier liner. It's a pain too, but actually has some real cold weather benefits.