FFG has very much managed to do what Games Workshop has failed in their normal gaming system - to really capture the gritty vibe of the grim darkness.
A couple of years ago I started a campaign which would span over a few Acts.
The first Act followed the exploits of heretics using Black Crusade rules, and plenty of havoc and mutation ensued over the next 12 months.
Act II sees a character swap into more loyal followers of the Imperium. It has now been going for 12 months, and with a few extra people it has really been a hoot. Act III will eventually tie up both the good and bad guys with some interesting results!
Here is a write up I've produced of the last four sessions we've held. Enjoy!