I just finished packing. Here is an example of what they helped me accomplish.
DD wears a child's size 7/8. In her carry on I have packed 5 very poofy thick custom sundresses (not typical thin Disney dresses but thick cotton with a lot of ruffle). 11 short/shirt outfits, 1 skirt/shirt outfit, bathing suit, 11 pair of underwear, 1 pair of sandals, swim goggles, and a couple other small items. The bags keep everything organized and do help to compress a bit.
I am an adult wearning a size 14. I was able to pack a full 8 days worth of clothing and an extra pair of shoes in a carry on.
One other perk. I have different colors for each person in our group. So packing different items into each cube and having different colors makes it very easy to stay organized in a small space like a hotel room. You can just plop the bags in a drawer, on a shelf, or whatever and have easy access to your stuff without having to dig through suitcases.
Hope that helps some.