So today I finally finished reading the Codex Infernus, the last chapter was about the new demons, other monsters, and other hazards from hell. It was for the most part a perfectly acceptable bestiary. The included lesser and greater demons were definitely intended for higher rank characters, especially if you are using more than one or two of them in an encounter. Pretty much all of them take only half damage unless being attacked with magic or cold iron weapons. The lesser demons are mostly what you would expect from this power level, shock troops, for the armies of the different more powerful demons. The greater demons are unique beings, in the built-in sample setting, they are the rulers of the various regions of hell. They are more interesting than the lesser demons, because they have personality and goals, but seeing as how they are more closely tied to the built-in setting example, the interesting bits would have to be changed before they could be imported into your home games.
I believed that the best part of this chapter was the section on Demon Lords, beings of such great power that they were not given stats, but just sections on their goals and what any of their followers might be doing in their name. These would be great things to use as the focus of any cult worship that you want to use in your settings.
Overall as I have said before I am really glad that I backed this book on kickstarter. While I felt like there were large sections that were good enough, there have also been some absolute jewels in the various chapters that I am looking forward to making use of.