Based on the DOOM engine, Heretic was only mildly popular on its release because there were so many copycat 3D-shooting games released around the same time. However, Heretic is one of the better shooting games released in the pre-Quake era. It has solid level design, good graphics & sounds (although of course you're limited to 320x200) and really the only major criticism that could be leveled at it is that it's too much like DOOM.