Just tried out the Pack-it 15. Most of the reviews I've read speak of sweaters and men's dress shirts which is great but I wear more dresses than shirts so I thought I'd post my observations. The 15 is meant for 7 shirts. I didn't want to put 7 dresses in the pack since they tend to take up more room.. they are longer than shirts!
I was able to pack 2 mid-length dresses, 1 short dress, 2 blouses and 1 thick cardigan in a pack with room to spare. The stack of clothes folded in a pile (pre-compression) was 4 inches tall. Once I strapped the pack down with the Velcro straps the tallest part of the pack (middle was thinner than ends due to Velcro being down the middle) was just under 2.5 inch.
I left the pack sitting in my room for a full day and when I unpacked the top dress had small creases on the sides where it over lapped but very minor. The rest of the stack including my silk blouse were immaculate.
Its good to note that my short dress measured 35 inches long and the skirt portion had to be folded over. So if you are intending to use these for dresses like myself, i would suggest getting the 18 Pack-it just for a bit more room (and less folding over).
Not to bad for my first trial. Real test will be in a few days.