I was looking at Bloom's extreme low light test and the C300 still has the dodgy Canon chroma noise that the Sonys and Nikons don't have. You have to push the ISO much higher to see it than you do on the 5D however. From the samples I've seen 6400 looks like 1250 on a 5D.
Today I ran (literally) around with some friends and 12 cameras in the worst scenarios, Epic, C300, GH2, etc.
I'll get a good look at the graded material after the DaVinci session with them all, but just from glancing... mmn... nah, Canon's own DSLRs don't have anything on C300 image. Down to the pixel. It's one of those cases where I'm pretty confident if people put their hands on the camera, they would see what everyone who actually has touched the camera is raving about.
That happened to me with the GH2 hack, didn't really follow all the noise until I had first hand footage and experience. If anything, it made me kind of sad that I don't have 16K to toss at tech that's going to be outdated under its own brand in a year or so. Also made me miss video cam format (built-in NDs and long-lasting batteries, easy media) more than I had realized.
Epic of course is winner overall, but the C300's a dream cam as well. 5D gets nowhere near in any department... sorry.
Seconding Pappas on the build quality, feels like a brick and it took all of thirty minutes to get around the menus without even touching the manual.