I use e-bags packing cubes and they work well. As everyone has mentioned their purpose is organization. If you are trying to pack as lightly and tightly as possible they aren't helpful, adding bulk and weight. You should definitely make sure to buy cubes that fit well with your bag. Measure the insides of your bag, and compare them with the measurements of packing cubes. I've noticed that with some bag and cube combinations the cubes leave odd amounts of space that I've had trouble fully utilizing.