This packing solution is different from typical packing cubes - each Pack-It has a thin polyethylene sheet on the bottom and a similar size/weight/thickness sheet that is "loose" to use as a folding board. When you are done folding your clothes, you place the loose sheet on top of your folded clothes, and then Velcro the top panels together tightly, effectively sandwiching your clothing between two thin sheets with a fabric wrapper around them. The Velcro system allows you to adjust the degree of cinching to match the amount you are packing.
The beauty of this solution is three-fold: unlike guessing where to fold your clothing, you have a clothing-retailer style folding board with instructions so that each item of clothing (shirts, pants, etc.) is optimally folded both in terms of preventing wrinkles and in terms of space efficiency; your clothing won't shift; and the plastic panels protect your bundle from the pressure or other packed items.
This solution is perfect where you have fairly flat clothing (pants, shirts, etc.) that you must keep wrinkle free. It is unnecessary and inefficient for smaller lumpy items (socks, underwear) and may be overkill for unironed clothing like blue jeans and polo shirts - assuming your luggage has tie-downs or panels.
Where the Pack-Its REALLY shine, though, is when they are used to add some much-needed rigidity to soft-sided luggage (large bookpacks, convertible suitcase/backpacks, duffels, etc.). With soft-sided luggage, the Pack-Its make up for their extra weight by acting as an "endoskeleton" for the soft-sided bag.
Even where the bag doesn't need much help with rigidity - a smaller Pack-It inside a day pack - it works a lot better than a packing cube because the Pack-It won't slump or shift.
The 18" should be your starting size. Unless you are small bodied, it will be hard to make the 15" work. The 20" is for suits, and won't fit some soft-sided luggage.

I have used both the 15-inch and the 18-inch Pack-It folder, but find the larger size to fold and fit my 16-1/2 x 36/37 shirts better. One can easily fit four long-sleeve shirts, plus another two short-sleeve polo shirts, and a light sweater, as a typical load. Ties can also be loaded between the layers of shirts, to keep them unwrinkled as well. One or two pair of pants then can be wrapped around the closed folder. For those who prefer to travel with an unframed carry on bag, this arrangement provides a "frame" for the case.

I was skeptical at first but absolutely won over. Now it is second nature to use this. The folder decreases the volume of the clothes and thereby increases room for other luggage contents or allows for use of a smaller piece of luggage and results in less wrinkles when care is taken in packing this properly. However the resulting density allows for packing more into a given suitcase and as a result a 28" wheeled luggage can, when fully packed, weigh greater than the allowed 20 or 22 kg international limit. Since the clothing properly folded and placed in the folder is the most efficient use of the space, the used, dirty clothes can be refolded and placed in the folder at the bottom of the stack separated from the clean clothes by the supplied folding board. This is the source of the only improvement I can think of for this item: 2 folding cards rather than one would be a help. Also, two folders might be better than one for long trips. On a 3 to 5 month sojourn requiring athletic, hiking, business, and casual clothes, I routinely fill the folder with 20 or more items (e.g.4 long and four short sleeve shirts, two pair long and two pair short pants, 5 boxers, one t-shirt, one Lacoste tennis shirt, one bathing suit and one pair gym shorts. When passing through warm and cooler climates, it would be better to separate the two sets of clothes in two folders. The resulting convenience may outweigh the increased use of space and increased weight. Also, when urban tourist backpacking, the folder can be removed from the pack in the hostel making all clothes easily accessible and much less rumpled than the usual backpacking experience. However, prior consideration of the backpack opening and the encroachment of the internal frame on the pack cavity will avoid a mismatch. This is an excellent product.