From what I understand, he's absolutely brilliant. The same with N.T. Wright. My brother in law is reading a book of his, not related to NPP or FV, and says the guy is heads and shoulders above most. He's a bona fide geemus.
I wouldn't call Doug Wilson a genius. He's not a dunce but his intellect does not dazzle.
I think his appeal is his folksiness as much as anything. I used to like reading him but I read another book recently and I just got tired of all the pithy "zingers". He tends to speak somewhat in aphorisms and that probably makes him seem profound to many people.