I find this to be the most complete of FFG's Warhammer 40k series. You can see where it has built on a solid system and is also a service to the fans.
Space Marines feel as devastating as they should, able to shrug off minor blows without a thought. This goes hand in hand with any space marine focused novel you have read. They feel right.
While Rogue Trader is a game about trying to get rich or die trying in a harsh universe and Dark Hersey is a game out standing up to forces beyond your power, Deathwatch gives you that proper feeling of being as superhuman as you should be.
I can't recommend this game enough.