HIPS, is not the prefered plastic to make armor, however, Its a cheaper alternative to ABS. Ive been told that it can crack and in some cases, actually cut someone. Honestly, it can be a flimsy plastic, again, no where as sturdy as ABS, but people do use it.
TK560 (Jim) has made his suit I believe from HIPS for his fan films. It also depends on thicknesess too.