This is a great book and my favorite interation of the BRP rules for a Mythos game and it does an incredible job of bringing personal horror to the Mythos and the impact that doing the DG job has on the characters.
In addition to this, it is one of the best books for information on US government agencies, both foreign and domestic, as well as information on the military. Even if you don't want to play an DG game, you could use this for a military or federal agent game.
Some of the best mechanical parts of the game include how Sanity and the Breaking Point works; Bonds that represent the relationships that PCs have with family, friends, and organizations that can be used as a source of strength, and will deteriorate over time as the PCs continue to hold back the dark; and Lethality ratings for weapons which is a percentage chance for some weapons to automatically kill a target (mostly human targets), and makes combat deadly which is appropriate for the setting.
Overall, it is a great new start for the Delta Green line.