A long while back, the head of Activision announced that their goal for many franchises is to exploit them on a yearly basis. Call of Duty is very much one of those series; the core of the game is, and always has been, multiplayer gaming with a single-player campaign sort of "tacked on" for new players to get their feet wet before jumping into the multiplayer arena.
Therefore, it should be no surprise that the Black Ops single-player storyline is short and serves mostly to serve up what amounts to a very linear spy novel. Players jump into the shoes of Alex Mason, a soldier in the Bay of Pigs invasion, and from there the action follows through a series of highly linear firefights in somewhat disjointed, disconnected environments - Cuba, Vietnam, Russian prison camps, and various city environments. In most of them, there's a "lead" character who's more or less invincible barking orders to the player, further leading them through the level to get to the various action or vehicle sequences. For those who enjoy simple linear play, it's probably just fine, but there's a serious lack of path choices for an action game.
On the upside, the AI in Call of Duty has been slowly improving, and between enemy AI that has differing strategies, to a few who will actually lie in wait and then charge the player rather than just playing "corners and cover" all day, it's a bit heartening. For the 4-5 hours of the 6-8 hour campaign (not counting video and vehicle sequences), it's welcome to see AI that actually exhibits something vaguely resembling individual behavior.
But, since this is a Call of Duty game, many players will never touch the single-player campaign. No, this is all about the multiplayer. To this end, there's a pretty solid set of maps set in campaign setting areas as well as some more unique ones such as Nuketown, a map designed to look like the facade-towns the US used to put way in the middle of nowhere to test the effects of nuclear detonations. Most of the maps are a bit more claustrophobic than previous games, closing in alleyways and choke points to make strategic use of certain newer items like the "camera spike" and traps work better; at the same time, the claustrophobic nature can be a real mess when playing with non-seasoned players.
Not to be outdone by previous titles, especially Modern Warfare 2, Treyarch and Activision have placed in a surprisingly large and diverse weapon set, and made it a bit weirder yet to unlock items for multiplayer - essentially, the class leveling, create-a-class, and game mode and weapon "unlocking" are handled automatically. In order to actually use unlocked weapons, players have to "purchase" them with "CoD Points", granted at a rate of 1000 per level-up. Once the points are spent, they're spent, which makes for an interesting way to level up and adjust a character's play style, but doesn't leave much room for rollback if someone mistakenly picks a gun they find out that they just aren't enjoying. Chances are, this is going to be a gameplay change that Call of Duty fans argue back and forth about for quite a while, especially with Modern Warfare's mechanics being fresh in mind.
CoD Points are also used for something called "wager matches", in which players "bet" (by "buying in") on their own success in missions with the hope of coming out with someone else's points. It's an interesting touch, but the cost of buy-in is a bit steep, especially when players will get better results in the other method for grinding up CoD points, a contract-style system where players invest a small number of points and bet that they can complete a given map objective for a larger return payout.
If you're not a hard-core player but still looking to get in on the action with your friends, Zombie mode gives the option to call up to three friends in to destroy the massing swarms of the undead. There's also a pseudo-multiplayer mode for practicing maps and tactics, in which AI stands in for humans, to give beginning players more practice before they jump out of the incredibly linear level design into the more diverse pool of online multiplayer.
If you're a Call of Duty afficionado or one of the types who wants to be playing what everyone else is playing in the FPS arena, Black Ops is for you. If you've got a friend you want to get into the game, well, Black Ops is a lot friendlier than previous entries into the series, and so it's probably a good starting point. Just don't expect the single-player to hold anyone's attention for more than a solid day.