He's one of the ten greatest position players in team history. He had a bad year in 2014, but otherwise his wRC+ has been ridiculously consistent. His issues have been entirely health related. And that is a reason to worry about his future. But when he has been healthy, he has been almost metronome-like.
He's no longer the best position player in the AL (his MVP was entirely warranted). But he's a clear 2-3 win player which is pretty good for his salary. Now with his knees I cannot bet on seeing that again, which is too bad.