I've been using these for the last 15 years and I think now more than ever they're an absolute necessity.  Not only do they allow you to pack more efficiently, but if TSA decides to empty your checked baggage during a random search your clothes won't be a wrinkled mess when you reach your destination.  I've never seen them take clothes out of these even when they tear your suitcase apart since you can see the clothes through the mesh front on the pack-it cubes.  Highly recommended.