Black: Almost perfect
The way the difficulties are set up is intriguing as well. Each difficulty gives you more objective than the last, and the reward for clearing the game with each difficulty is well worth it. Especially Black-Ops mode.
Action is almost non-stop, and really the only time there isn't any is if you're exploring areas for other objectives after clearing out all the enemies in said area.
There are a couple missions where you have team-members, but they're all pre-programmed to do their own thing and go their own way, and never really deviate from it. But they're very useful when they do show up, though you can be cheap and let them do most of the killing; they're apparently invulnerable to everything, including explosions. They're a 50/50 thing.
Some disappointments:
It had very few missions, although I suppose it makes up for it with how lengthy they can be.
The availability (or lack-of) of some weapons was frustrating. The P90 makes an appearance, but only at the very end of one mission, and wasn't even worth grabbing when compared to your starting weapons, for example.
The ridiculous amount of armor your enemies have can kill the enjoyment sometimes. There were a couple instances where it took almost two full M16 clips just to down someone, and a half a clip to go for the head. Although it's understandable why and how they can get this armor.
The ending was the worst disappointment of all...but I can't go into much more detail without spoiling it.
Regardless of the negatives, it's definitely a game worth investing in.