I love Nazi Zombies! Me and my cousin Gep got to round 32, and I reached 200 solo. (I glitched for a little while on the solo 200, but not on the round 32 game.)
farthest I have gotten is level 28 legitimately with a 4 man squad. They recently fixed the sand bag glitches, and bipod glitches, and even attempting the bipod glitch will get you thrown from the game. Fun, but gets very boring and monotonous, especially with a 4 man ray gun team with mg's and 1 flame thrower.
The new Zombie Asylum map for the map pack (3-19-09 on PS3) looks very awesome, and seems to be worth the presumed $10 cost on its own. Definately cannot wait until thursday!
I play it at some chaps house when I get the chance, but I really enjoy it. It's a fun shooter style game, especially if you make the zombies 'crawlers' by blowing off their legs.
I've gotten to level 28 by myself and with another chap of mine, we reached level 50. It was very crazy and very spookys as the zombies appeared out of nowhere.