I'm no expert on the Warhammer 40,000 universe. But I know the basics: totalitarian space empire ruled by oppressive theocracy which worships psychic emperor in life support, fights a dire war for mankind's continued existence, against aliens, mutants and heretics worshipping extradimensioinal demons.
I like Dark Heresy's "investigative horror in a dystopian far future" milieu. It seems to me to be the easiest to grasp and run right off the core book.
I also like Deathwatch's "black ops Space Marines" premise, which is just so promising, but I'd be leery of sliding into a repetitive pattern of kill-fests. Not that the occasional kill-fest game isn't great.
Rogue Trader I'm not entirely sure about. It kind of feels like Traveller on crack. Which I'm not entirely sure whether it's a good thing or a bad thing.
So, what do you think? Actual play experiences welcome.