If you're still wondering whether or not it's worth working with an external HDMI recorder with the new Canon 5D Mark III firmware, this excellent video from Vimeo member Stefan Czech should answer any of your questions.
In short, you may not easily see a difference from the file recorded from in camera when comparing it to the recorded file from something like the Atomos Ninja 2, but it will make a significant difference as you start to adjust colors in post. Another benefit of course is running longer than the 29 minute video limit when recording from the HDMI output (great for guys who have to leave a B-cam rolling).
The Atomos Ninja 2 is not just an external recorder, but also doubles as a pretty handy external monitor. New lowered prices were introduced during the NAB 2013 hoping to make the unit more affordable to 5D Mark III shooters who want better quality from post (found here).
Atomos Ninja 2 HDMI Recorder 5D Mark III