I just played it solo with two controllers plugged in. Kinda annoying that the people behind the demo force you to do that. I mean, they must know that most people aren't going to have a buddy sitting right next to em conveniently. So all it does is force everyone to play it using two controllers. Really retarded, if you think about.
On normal, it seemed really easy. I was just killing all the enemies by myself. Never felt much of a need for a second player. Weapons loadout menu is pretty interesting, there are a fuckton of options available for customization.
Graphics looked really nice. Seems like it'll be a good purchase even without the co-op.