My brother and i tried the demo, then got the full game to try CoOp.
We have quite a bit of lag while playing, it's odd lag though. If we stay moving it seems pretty fine, but stopping and starting has a delay on it. For example i'll see my brother in the jump animation hovering just of the ground.
I was assuming this wasn't normal for this game, cause it probably shouldn't be lagging over a lan connection.
Made sure it's not being blocked by a firewall, forwarded ports, messed around with the maximum transmission units a bit.