Well, underrated in sabremetric circles, anyway. I know what you're all going to say too. He had a 4.17 tRA last year - above average, sure, but worth nothing close to the $18+ million dollars the Cubs spent on him. In fact, his pitching WAR (tRA based, not FIP), was 3.3, which would value him at around around $15 million.
An overpay, then?
Only if all you do is look at pitching statistics. Zambrano's true value was actually more than $20M. How?
Pitchers in general are pretty bad hitters. Over the last three years their totals have looked like this:
That's (not) good for a wOBA of about 0.154*, well lower than league average. In fact, over 700 plate appearances, the average pitcher would be worth -11.5 wins with the bat compared to the league.
In this case we'll take average as replacement level - pitchers aren't selected for their bats, so there isn't a very compelling case to further widen the gap. In addition, pitchers won't come anywhere near 700 PA, so that enormous replacement win value will drop considerably. It's still significant, though.
Over 85 plate appearances in 2008, Carlos Zambrano was worth 2.5 runs over the average hitter. 85/700*11.5 gives him a 1.34 positional adjustment for his batting, which marks his overall production as a hitter at about +1.6 wins.
1.6+3.3 = 4.9
4.9*$4.5M = $22.0M
Zambrano's bat was about one third of his overall value. Neglecting pitcher hitting is a great way to miss out on the big picture.
PS: I imagine Matthew will come along shortly with pitcher hitting WAR for everyone not named Zambrano.
*I did a cursory check to make sure this isn't being skewed by AL pitchers in interleague; using NL pitchers only gives a similar result.