One of the most underated FPS of all time.
It has awesome gameplay, lots of original weapons (like the Flamewall, Drunken Missiles, or Split Missiles, not to mention the Heat-Seeker!), many, many enemies, and very challenging traps and bosses. Oh yeah, and a significant ammount of Easter Eggs. In my opinion it isn't below Doom 2, the "hit" of that time.
If you're a hardcore FPS fan, and you played games as Doom and alike, you should give a try to this game. You might find it on abandonware websites. Or download it somewhere else. Just do it, you'll thank me later :D