Here in New York City it seems you're required to use metal boxes by default, since use of armored cable is apparently a code requirement.
Although I've seen the blue plastic boxes for sale at the Home Depot...what they're used in I don't know. Everything I've seen/dealt with has always been metal (which I prefer since they're much sturdier - plastic always can crack or the screwhole strip out if you over-tighten).
You cannot use armored cable with plastic boxes, only NM, right?
