I don't know that words can accurately describe how surreal an experience it is to play Doom with the sensibilities of Mega Man.
ScrewAttack Gaming Convention attendees had the opportunity to play Mega Man 8-Bit Deathmatch recently, and now the rest of us are getting a chance -- creator "CutmanMike" has released the current build onto the Internet. Plain and simple, you owe it to yourself to give the game a download.
Need more convincing? Go back and watch this trailer Jonathan posted a while back. I can practically guarantee it'll give you a seizure; I mean that in the best possible way.