I was always very skeptical of these for doing crunches, I just didnt feel it when I tried. I hear the same from others.
Then I gave it a fair chance and after a few ab routines I figured out how to use it properly and it has since worked great for my Abs, other than lower ab area excercises, its all I use for crunches now. And It makes it easy to watch the News while im doing it! I dont use it for DB presses as of yet, unless I do it in an Incline press manner, It feels like im going to roll backwards. I have used it for DB flyes on occasion and works well.