Classic example of one step forward and two steps back. Simple mistakes could have been avoided, still decent with live.
The good: Fantastic customization, multiplayer modes are well created and just amazingly fun, much better system for unlocking mods, maps are interesting and fun to play in. I highly recommend trying out the wager matches if you're the type that doesn't care about losing a little cash. Sticks and stones and gun game are probably my favorite multiplayer modes I've come across in awhile, and I'm glad Treyarch thought of it.
The bad: Single player campaign has significant issues, especially in some levels like "Defend Ke Sahn" which is infamous for being the stupidest video game mistake ever seen, including the mistranslation of all your base are belong to us. Instructions pop up saying, "Push back Viet Cong" and that's it. You sit there for twenty minutes trying to figure out what you're supposed to be doing until you finally decide the game is bullsh*t and you google it. Turns out you're supposed to be going up to unmarked barrels of napalm and kicking them down the hill. Very counter intuitive, highly frustrating moments like this ruin what could be epic moments of the game. Other minor frustrations include keeping last stand in multiplayer, and some minor balance issues that you notice in multiplayer (some guns are just merciless).
The ugly: the engine used is different and it's obvious the moment you start moving. Compare to MW2 and you'll notice that everything from walking to sprinting to reloading your gun feels chunkier. It doesn't run as smooth. Speaking of running smoothly, I know a lot of people who have had connection issues with Treyarch's servers and get booted randomly from matches or lobbies. To make matters worse, Treyarch didn't plan ahead very well and decided that one error message would suffice to cover a broad range of errors, making diagnosis nearly impossible. Combined with the terrible service from Activision (because Treyarch won't deal with you directly) it means that you have a real issue with the game you might as well get used to not playing it. The other major issue with multiplayer just may be fixed with the map pack. The new DLC added a couple new maps that are designed with snipers in mind because let's be honest here, they didn't bother making any. There are a couple spots you can snipe in a couple levels of the original maps but those areas are well known and if someone is using a sniper rifle he is being taken down fast. Snipers have been significantly powered down for black ops making them a less viable play option for elitists who take pride in shooting out your left eye from 9km away.
And probably the ugliest problem of them all: the lag issue. Everyone notices it, Treyarch officially denies it as a problem to avoid dealing with it. When you kill someone they keep running and actually are able to make it a couple steps and fire a few bullets before dying, even though they've already been killed. This is another problem with the engine Treyarch used. Treyarch reps have denied it's an issue, claiming that millions of gamers are simply mistaken and that it's our connections, not the game. Compare black ops to MW2 and see how the kills work. When you shoot someone in MW2 he goes down instantly. Black ops has an undeniable lag time that's obvious to any player, new or vet, but Treyarch's unsupportive response left many players upset, and for good reason.
Overall it's a good game. It's a breath of fresh air from MW2 which has been played the hell out of for a long enough time now that it feels good to go in and try out a new system and new maps. But MW2 was a powerhouse of an FPS, and black ops was the sequel and had high expectations because of it. Some issues, (the lag problem, terrible support, deliberately making sniper rifles nearly impossible to use) aren't excusable in any game and should have been fixed before the game was released. And overall, Treyarch picked the wrong platform to build Black Ops on. But give it a try, most likely you'll be hooked despite the flaws.