There's two different arguments taking place in this thread. One is that Kallis is an underrated all-rounder overall, and the other is more about how he's been performing in recent years.
I do believe Kallis is probably underrated by many (including myself), and as Jono has argued before, it's probably largely due to feelings of dislike he inspires in many cricket fans, due in part to the perception that he's selfish (as well as "cashing in" with the ball against minnows in recent years). I think there's evidence out there to argue occasional selfishness - others may argue that he merely puts a high price on his wicket, and I wouldn't argue that it's bad to do so in of itself, but obviously it can be a problem if the price of your wicket is higher than the prize of actually winning games.
Anyhow, his career stats overall are very impressive, and as far as allrounders go, he probably is a tad underrated by many. I do think many people aren't that fond of cricketers/sportsmen that appear overly "opportunistic" though, at least in that way.