rewards for finishing are:
normal - don't have to wait before throwing next granade
hard - press 'F' to blow up grenade anytime
elite - unlimited grenades
After 10 seconds of continuously shooting a stationary enemy with the minigun he's still alive and runs off. Then takes over five shotgun shells and quits because what's the point?
Early levels are too easy and I get bored before getting to anything more challenging. I suggest skipping the early levels after learning the mechanic.