I didn't know which one you were talking about, but the first game is actually not that great imo, and 2 is the peak while 3 was a disappointment, so I'll give the series as a whole an 8.5.
Black Ops 2.
I've considered buying a PS3 time and time again, purely for Little Big Planet, God of War and Uncharted. They all look so good.
Black Ops 2 is a good game, but i'll give it a 7.5/10, they really haven't improved a whole lot since COD 4, and until they do, they shouldn't receive excellent reviews. (saying this I haven't played the campaign which I understand has been revamped in some way), BF3 came out earlier and the gameplay and mechanics crossed with better size maps and an added element of "real feel" really make Black Ops 2 not look incredibly special.
Half Life 2