Zambrano's Cub career is over, and while his legacy is certainly tarnished by the incidents that eventually drove him out, his contributions were huge.
Statistically speaking, Zambrano was one of the greatest Cubs pitchers of all time, and one of the 3 best of the modern era.
Using bWAR I totalled up some of the names that have been thrown out as the greatest Cubs pitchers of the modern era. The first line shows their total (pitching only) with the Cubs, the second their career, and the last their bWAR as a hitter. And it's not just longevity that puts him there - when you avergage it out, Z's WAR / Year as a Cub (including batting) is higher than anyone not named Fergie.*
For the traditionalists out there, here is how the starting pitchers among that group compare in terms of wins:
People love wring their hands and saying that Z never lived up to his potential, or wasn't worth the trouble. I'm not saying whether or not that's true, but objectively speaking there are few Cubs pitchers any of us have seen in our lifetimes that were better than Zambrano.
I for one will miss watching him play baseball.
* Black Eyed Peas excluded