I spent 4 years in a light infantry unit and I have nothing but great things to say about the poncho liner. They are extremely warm considering their size and weight. I would also like to point out that we never actualy acctached the liner to the poncho, we would simply wrap it around ourselves.
That was many yers ago, so I am sure that the new thinsulate liners are much warmer for less weight and size.
Additionaly, if you do have a sleeping bag, the poncho liner will increase its effeciency if you combine the two.
Hope this helps.