Call of Duty has, for recent years, been among the most played multiplayer games on Xbox LIVE, and Black Ops will no doubt continue to maintain similar popularity.
I've logged about 42 hours into the multiplayer so far, and while not too much has changed, the game remains as fun as ever. Unlike Modern Warfare 2, this game - in my opinion - has great maps top to bottom with maybe one or two exceptions, which is really great. [See my review of the maps in my past post.] Most of the changes/additions to Black Ops, like extra gun camos, recticle customization, emblem creation, and so on, don't change how the game is played, but do add a little extra fun to the multiplayer, I guess.
Black Ops LIVE has very few flaws. Fortunately, with Treyarch, you can trust that when a problem arises, they'll get off their asses and do something about it ASAP, unlike Infinity Ward. Some glitches worth naming have now been patched, leading to this waste of a sentence.
Trying to keep this review short, I'll go ahead and talk about Zombies - I hadn't played too much WaW, and didn't touch Zombies in it at all, but it never REALLY appealed to me. Black Ops has two Zombie maps on disc - one during World War II, and another to be unlocked after finishing the campaign. [Future Zombie maps are very likely to become available via DLC.] While I don't find Zombies awesomely fun, it is entertaining for about a half hour/hour at a time. But hey, to each their own. Future map releases for the Zombies gametype may change my opinion, but as of now, I only really enjoy the "Five" Zombies map. [The one unlocked after beating the campaign.]
Dead Ops is also available to play once you unlock it. [See YouTube if you don't know how to unlock it yet.] I guess the best way to describe Dead Ops is to compare it to Contra, but instead of being a side scroller, it's played bird's eye view. It's fun, and plays a lot like Smash TV, if you've ever played that game. The game looks good, there's a lot of Zombies to kill, and it's really a fun little addition to the game.
Overall, Black Ops - to me - is the best Call of Duty since the original Modern Warfare. The game feels more balanced, and despite a small handful of flaws, is much, much, MUCH better than the mess that was Modern Warfare 2. If you don't already have the game, pick it up as soon as possible.
[Few minor flaws keep it from being perfect, but they should be fixed in future updates ... Map packs will also give the game more variety.]