And I definitely agree that Colin could have made an AMAZING Doctor if he hadn't just been written as "Mr. Crankypants." That stuff all made sense in the Twin Dilemma, but then he never really developed beyond it. And Colin himself, I think, would've been both game to play the role differently, and entirely capable of doing so, if given the option. But it just never happened.
I've heard he's really good in audio books. I've never heard any of them though.