Chili is right. I have some plastic boned stuff, but they aren't meant for support or shaping or waist training. They hold a little shape, but they twist and can break. It's OK if you aren't trying to take off too many inches, but if you want FULL support, go with metal.
I have some metal boned cinchers and others and they HOLD you in, but you do need a lace up back to take best advantage of metal boning. Zippers or hook and eye closures won't hold you in properly.