Okay so this is my first break from the game... already logged around 20 hours co-op into the game.
I've played 4 players and 2 players. Although lately the other two people have been getting involved with school and stuff, so it is left to me and one of my good friends. The campaign is a lot longer than I expected, it is 20-30 hours, maybe even too long if that was possible. The story has some... interesting turns and hilarious cut-scenes, quests, and stuff. But, man this game is a lot harder than the original Borderlands.
We had to fight a boss *Bunk3r with like 1,000,000 hp and we only did half damage to it because it was two levels higher than us. The boss fight actually took us approximately 35 minutes, my mate actually timed it XD
Most enemies can down you fast, and bosses/badasses/super badasses/Ultimates enemies can either three shot you with an SMG or one shot you with a sniper or rockets. Oh man super badasses make me cry so much. I couldn't imagine a super badass Constructor... oh God. I really can expect to be flimsy because I'm a medic siren whose got around half health and shields as my commando buddy.
Anyway... I really think this game is awesome with co-op and average in solo. We kinda gave up on the main story for the moment because the enemies are 4-5 levels above us, but I think it is going to end soon because of the whole *Waking the Warrior* thing. I'll probably finish the campaign soon though. Borderlands 2 is an amazing game.