Just like the Low D extension, I didn't learn about the High C extension opening until the day after it opened. I explored it upon learning it was open, and it's pretty nice.
The High C extension feels like an extension of Terminal C (but much nicer), and stores/restaurants are in their own rooms.
This is opposed to the Low D extension, which feels like it's own thing far away from the main building and where stores/restaurants are out in the open.
The High C extension has a "Fort Worth Magazine" store that's not open yet. Though, I believe it's just another Hudson News or something similar branding itself as a local news store.