I played a Black Crusade demo. It was very fun playing Evil PCs and doing the whole tragic villain (or psycho villain) thing.

My character was a fallen priest who calls upon the Chaos gods to help the Imperium because the Emperor doesn't understand he needs more assistance to fulfill his dreams. Tzeenth has the power to evolve the human race. All we gotta do is use Tzeenth's powers for good...

Our table also had a Khorne Beserker which didn't make canon sense, but we went with it and he predictably led us all to inevitable slaughter and destruction.  

Good times. I would definitely run it with the right group.