I honestly didn't mind the flood levels in Halo. It felt like Doom. All dark, simpleton enemies everywhere. Everything else about the game felt different to other shooters, so it actually had a nice touch of familiarity to it.
And no flaws could bring the game down from highs like dropping on the beach on Silent Cartographer or the whole of AoTC. And just romping around that pretty weird world in the warthog, god damn.
I'm getting all emotional and stuff.
I enjoyed the change of pace that came with fighting the flood in levels like two betrayals and the maw where you alternated between them and the covenant. Variety is only a good thing imo.