Shane Hurlbut recently shot an interview for a Game of Thrones promo with the Canon C300 alongside the Arri Alexa. In the above video, you can see some of the interview shots cut in with footage from the show (the show itself is shot on the Alexa, if I recall).
Below, you can see some side by side shots (no audio) of the same interview scenes with the Canon C300 and Alexa.[youtube http://www.youtube.com/watch?v=Pneu6CDBFUk&2=640&h=360]
Loads more details on this comparison over at the Hurlblog.