No doubting his capabilities with the ball, it speaks volumes of how well he's done when it's a big talking point that his average has touched twenty!. Anderson, Zaheer, Broad are (were in Zaheer's case) are considered good and they average 30! However, as others have pointed out, the average probably flatters him a bit. If there's one complaint I have against him it's that he seems to be pampered a lot. Smith always seems to turn to Morkel, Steyn (and Abbot in the last match) when there is dirty work to be done and when the going gets tough. That's probably good strategy more than anything else because Philander is not as effective with the old ball as he is with the new ball but still...I suspect that if he had been part of an attack like say, India's or Sri Lanka's, his stats perhaps would be worse (not Ishant Sharma type worse but maybe three-four points or so more than he does now). Pure conjecture though, gun bowler. Waiting to see how he goes in India and Sri Lanka.
On a related note, how underrated is Harris? Averages just a couple of points more than Philander and IIRC he was gun in Australia's win in SL 2-3 years back.