Nesta is the best CB I've seen and he's put in performances this season against Barca that I doubt anyone else in the world could put in just now.
However, I wouldn't sign him.
Too injury prone and he is right at the tipping point just now - he can still put in the odd great performance, but you can see he's having to put himself under extraordinary strain to stay at that level.
Given his physical state, I suspect this season will be the last one in which he is able to put in great performaces and you'll see him go downhill next year and look very ordinary most of the time.
I hope he retires at the end of this season so that he goes out when he can still just about cut it at the top level rather than spending another season or two in decline.