First of all, did you expect any less than three hours from Peter Jackson? I mean, he's currently the king of Epic.
Honestly, I dig Jack Black, and I loved the movie. But then again, three hours, four hours, ten hours in a theatre, or watching any flick/stage production that gets my interest doesn't bother me at all. And I have a lot of interests ;-) ...... I thought Jack Black did a superb job with the role, and I have the priveledge to say that I can submit such an assessment as one who's studied the Art of acting, as well as theatre in general. So there. =P