The Flood is the worst excuse for an enemy type in any FPS game.
They slow the game down considerably.
Halo 1 was just brutal with the Flood. The Library level has to be the worst level ever.
The only part I liked about the Flood was the open sequence in Halo 1 where you start going down the levels and seeing all the shit that happened and the eventual attack. That was pretty neat, but Bungie ****ed it up with generic level design afterwords.