I was an initial doubter when it came to these packing cubes -- packing cubes in general, actually -- but I took a leap of faith and decided to try them.
 For context, I will include two pictures I took this evening while I was packing for a 4-day trip to Miami; I used the two largest packing cubes and a bag that meets the maximum dimensions for a particular budget airline's personal item: 20 x 11 x 9.

The first picture shows everything I would have packed without any regard to luggage size: 2 dresses, 1 jumpsuit, 1 romper, 1 pair of jeans, 1 pair of capris, 1 pair of shorts, 3 t-shirts, 2 swimsuits, a cover-up, pajamas, 6 pairs of undies, 2 pieces of shapewear, 3 pairs of shoes, jewelry, sunglasses, 1 medium clutch, and all of my toiletries (makeup + contact lens stuff, etc).

The second picture shows my packed bag next to the items I had to take out: 1 dress, 1 bathing suit, and one pair of shoes.  THAT'S IT.  Everything else fit snugly in the bag, and while it wasn't effortless to zip closed, my toiletries are easy to reach for TSA purposes.  I am so amazed by this spectacular feat of packing that I will NEVER go without packing cubes again!