I'm going to start out with saying this is an amazing game. Totally in line with Borderlands 1&2... and destroys Borderlands Pre Sequel.
However! Having said this... Split screen co-op is, in a word, bad. The fact that I can play couch co-op with my son is a HUGE attractor. Unfortunately Gearbox dropped the ball on this. If the battle is hectic? If your partner is in their menu? If your partner is in a vending machine? You lag up to 3 frames per second. It's not unplayable... But it's not good.