It is very different from MW2, but it's still very good.
Campaign
It's very different from any other game I've ever played, and I found it hard. I first played it in the Regular difficulty and in some parts I had to search for an walkthrough in the Internet because I just didn't know what to do or because I got stuck somewhere. In some points, the objective marker isn't very clear and you have to ask for help, but it only happened once for me so it's not a big deal. A lot of people complained about these things and about some bugs, and I agree with them. Fixes should come ASAP. Mostly, the game is very dark, I even had to change the brightness of my TV when I first played the game so it would be in the recommended settings, and it feels a little wrong because in some parts an enemy will just come out of the shadows and kill you. The story of the game is really interesting and it makes you want to play more just too see what happens, but it's very short. Based on the ending, I suppose there will be a sequel although the gameplay and story will be different.
Multiplayer
Honestly, I think that from the 360 million dollars the game earned in the first days, more than 200 million came from the previous games reputation. The entire Call of Duty series is known for having a great multiplayer and Black Ops was expected to have an even better multiplayer because of the new additions, such as CoD Points. Honestly, I found the multiplayer very bad comparing to MW2. The CoD Points are a fun new addition, and Wager Matches are very fun too, but sometimes it can get annoying because you need money to buy a gun and it might be expensive. This probably will happen only in the beggining though. Since the game just launched, a lot of people are experiencing this problem. The multiplayer is still good, but if you have MW2, don't buy Black Ops if you only want the multiplayer.
Zombie Mode
First, let me say that I haven't played World at War, and the only zombie mode I've played before Black Ops was the iPhone game. And only Nacht der Untoten. But still, from what I know, the new Zombie Mode is as good as the old one, and the 2 new maps and the Dead Ops Arcade easter egg are very cool. Kino der Toten is the map you have since the beggining and it is a big map where it is easy to get lost. Personally, I have trouble finding machine guns but it's probably just me. It is great to play though. "Five", the map that you unlock when you finish the campaign is very different from all the others maps. It's small and hard to play because it's filled with tables and stuff, so people usually are overwhelmed by wave 7. Since "Five" is some kind of related-to-the-story-but-impossible-to-happen-according-to-it map, I shouldn't talk too much about it, but it's VERY funny and the characters usually say cool stuff such as "HEADSHOT! Try to shave now!". Finally, Dead Ops Arcade is an easter egg that you get by typing DOA in a computer. It is one of those old-school arcade games and it's great to play if you are in a get-away-from-the-darkness mood.
Overall
The game is worth buying. It has a great story, although you discover more about it in one of the hardest campaign modes I've ever played, a solid multiplayer which is very competitive and addicting and an awsome Zombie Mode where you can have a great time with your friends.