Heavy Melee is pretty incredible as an Infiltrator. On a decent difficulty, enemies will shoot at you if you try it as any other class. And when you get shot you slow down. So they can shoot you more.
Still, it's kind of disappointing. I was hoping it was an actual melee system since every class has a unique melee. Sentinel has two blades, for instance. But it's just a graphical difference. I had vague fantasies about charging at the bad guys just going to town. Not charging at a guy, holding the button, and waiting for the animation to end.