A two edged sword this one. Much like Ninja Gaiden, it will punish you for daring to play it, but you can't help but come back every time and try again.
Right now, I'm a level 40, with The Maintainance Room Key. I've gotten the failed ending, the ending where you escape without getting the whole story, and the one where you miss the helicopter. The only reason I haven't finished the game and gotten all the Case files is because I tried to unlock the Megaman Gun. You need just under 60,000 Zombie kills, and I racked up 40,000 just before the game ended. So now I have to start again