The majority of the book details the default system setting, the Gilead System. The planets all have several sidebars with adventure hooks, and the factions and patrons have lots of info such as relationships with other factions and sample mission types to help the GM use the lore information when planning an adventure. It all seems much more useful than just lore information, however if you aren't planning on running a game in the Gilead system you might not get as much out of these sections.
The character options are welcome additions. It is good to have lower tier options for techpriests and inquisitors, and the additional space marine options are necessisarry if you want to run a purely space marine game. However I'm still waiting on the Adeptus Arbites to make an appearance.
I always like to see games add downtime rules, and the Endevours rules seem simple enough that they won't get in the way or take too much game time to resolve.
The main thing keeping the Forsaken System Player's Guide from being a 5/5 for me is that it needs some more editing. After my first readthrough I noticed several errors or just confusingly worded paragraphs that need fixing. I'm going to give Cubicle 7 the benefit of the doubt that they will use their errata submission system to good effect and these issues will be fixed in the future.
Update: I am going to change my review from 4/5 to 5/5. After an update my initial issues with the editing seemed to all be corrected. I'm glad to see a quick update and the errata feedback system put to good use.