I also wanted to confirm that Mac OS 10.9 Mavericks works fine in terms of tethering the Canon 1D X. Tethering works in both Aperture 3.5.1 and Canon’s DPP v3.13.45.
A reminder, too, for all programs running in Mavericks where you need them to run in the background (e.g. FCP X, Aperture, DPP, Photoshop, etc.,) be sure to turn off “App Nap.” Go to File Menu > Get Info (command – I) and select “Prevent App Nap.”
Updated 12/2/13 – Corrected C300 firmware number and link.
Update – 1/18/14: The video seen in the BTS shot above is now up – All for One and Supercuts for All.
Update – 3/17/14: My take on the firmware: Canon C300 News – Dual Pixel Coming and More (about mid-way through the post.) Also, tied into the post above, I was having issues with Aperture and 10.9.1 on my Retina MacBook Pro when using the loupe. It would quickly crash Aperture. 10.9.2 has fixed that (most likely a memory leak of some sort.)