I've only ever seen one bad Robyn Williams movie, and heard about one other.
He's an amazing comedian (I saw him in a San Francisco comedy bar with 40 people in it before he started doing movies), and what is more amazing is how well he can act. It would have to be a particularly bad movie to flop with him in it.
Cheers.
p.s. I've been topping up on DES lately ... cheap as chips