I've only opened the codex a couple of weeks ago, not sure where I'm at today. This seems the most relevant place to say that this is a great change to the game. Getting most of the irons on my mission log was a fun task for me, but that's only because I did it as a separate task (get all the missions). Having to get them, make the exchanges, etc... was basically just chores.
The codex provides a real sense of progression, and a sort of positive reinforcement that's very encouraging when you're exploring around and start picking up mobs. The automatic achievement/reward cycle is good.