Knifing while holding a weapon seems pretty dodgy to me, sometimes I'll get them and sometimes I won't. Running around with just the knife out though seems to work a lot better as you can lunge more frequently. Haven't ran around knife-only myself, but I've played against some to know how annoying it is.
If you are referring to zombie mode, there is a definite difference. I mean it was great in Black Ops I zombies, but in this game it reminds me a lot of WAW's knifing in zombies, which wasn't good by the way.