I was extremely hesitant to pick up Doom until I learned this mode supports co-op. After playing Onslaught, I'm extremely eager to see what the community comes up with after it can spend significant time on creations. This really is a feature that they should have been advertising more heavily, because it really got lost inbetween the "campaign" and "multiplayer" hype. It's also rare to see map editors on consoles that allow for this type of granular modification to gameplay, which is extremely exciting to see.
Yep. I've continued to play and have come up with some great co-op puzzles where players are forced to seperate into smaller teams and then placed in difficult situations where their success or failure makes things worse or better for the other team.