Compression cubes are sold with a before-and-after photograph and a percentage. The percentage is real. It is also measured on bulky knitwear, which is the most flattering possible test.
The saving is real, and it is very uneven
Compression removes air, so the saving scales with how much air the garment was holding. On fleeces, hoodies and chunky knitwear you can genuinely reclaim a third of the packed height. On jeans, chinos and anything dense you will see almost nothing, because there was little air to remove. Averaged across a realistic wardrobe the number lands closer to fifteen or twenty per cent than to the fifty on the packaging. That is still the difference between closing the lid and sitting on it, but it is worth buying with the honest figure in mind rather than the marketing one.
What you pay for it is creases and weight discipline
Compressed fabric creases, and the more you compress the longer it takes to hang out. Linen and crisp cotton shirts should go in an uncompressed cube or a garment fold; knitwear and casual cotton do not care. The other trap is weight: compression changes the volume of your bag and not its mass, so a case that now closes easily can still be over the airline's limit. Freeing up space tempts you to fill it. If you are flying an airline that weighs cabin bags, decide what the bag weighs before you decide how much room you have left in it.
Use compression on the bulky half of the bag and leave the rest alone. That is where almost all the benefit lives.