Round 25 is very easy with slow spawns and does not take very long to get to, wall guns with double tap are still very effective . Round 70 is very different from round 25, the spawns are much faster and the zombies are likely to die out and re-spawn before you kill them with a wall gun.
Fatigue becomes a big factor when you hit the high rounds making it more fun in a way because you are more likely to make mistakes. I don't really get bored if I have a friend in my party to chat to but I must admit I was ready to stop playing zombies when I played mainly solo and with randoms.