I have two that I use. The first is a Sea to Summit inflatable that is very small and lightweight and takes up very little space in a pannier .....but..... it is not nearly as comfortable for me as my other pillow, a Thermarest that is comprised of foam. The Thermarest though not really heavy, unfortunately takes up a lot of space within a pannier even when rolled up and compressed, which is Ok if I'm running both front and rear panniers but all too often I make use of only 2 panniers and therefore I need to be more efficient.