I think they are alright, they seem to be popular. Personely I'm still a wood liner guy, haven't had a truck without a wood liner in 30 years.
Last truck had Line-X, after ten years I was pretty impressed so it was the first thing I did to the current truck. Don't think I'd have a truck without it. I've always wondered about water and stuff getting trapped under plastic or wood liners and rusting prematurely. Have you guys ever had any issues with that?