I cant imagine not using a dressform when I make my costumes.
It helps to see proportions, and you can visualize the whole costume as it would look on a body, and not flat.
My dressform is .. supposed to be sized to me, but I have a really long torso, so I cant use it to fit things like dresses or long shirts.
It also doesnt squish, so if Im making and fitting something that needs to be tight, or is stretchy, I cant use it.
Ive wondered about making one with the duct tape method, but Im curious how it works with pinning. I pin things to my dressform like its a pincushion, and Id think the tape adhesives would pose a problem..