I got to tell you, I hate the multiplayer part of Black Ops which is why I returned it to get MW2. But the zombies is awesome. Whenever I go to a friend's house, we play zombies. It starts out slow, but after you get past round 15-20, it is loads of fun. I remember when WAW came out and I torrented it for my PC, and after I beat the campaign, I still played zombies for a couple of weeks, just by myself. It is not scary, as Treyarch wanted it to be, but it still gives you a thrill. I even got the app for my iPod and bought all the map packs. Let me share some records:
On Nacht der Untoten I got to round 29 solo, and round 46 on the iPod version with a friend.
On Verruckt I got to round 32 solo, and round 39 on the iPod with a friend.
On Shi no Numa, I got to round 22 solo, and with a friend we got both the Wunderwaffe DG-2 using an easter egg, and we could pretty much go on forever, but we stopped at level 50, due to getting bored.
On Der Riese, solo I got to round 19 (so good :D) and with 2 friends I got to round 40 with friends.
On Kino der Toten, I got to round 37 solo, and round 37 multiplayer with my friend on Black Ops.
On Ascension, I got to round 18 solo, and round 36 with 3 friends.