I'm only second prestige. I never really cared for rank in Call of Duty games, as they only correlate to how much you play. That being said, it's the unlock system that's truly addicting, and I like the trend of prestiges actually giving you rewards.
As for Zombies, I have a single knife. I don't like how you lose your tallies if you're inactive, because I can't play all the time. Again, it's hard to care about rank here when it's really your stats that are going to give a better idea of how good you are.