I've run Deathwatch a couple times, but find myself more drawn to Only War. It seems more flexible in terms of personality and origin stories, and I tend to identify more with soldiers than supermen.
But I ran into a problem when I tried to pitch it to my usual gaming buddies - no one wanted to deal with the high body count and human wave tactics. My friend Justin asked if he should "name his character or just give him a serial number;" his brother Nick recounted a story where a GM had wiped an entire group when a melta went off inside a shuttle. Evan didn't want to play because he hates Imperial Guard with a passion. My brother Stephen said that he didn't care for the 40k universe as a whole, since there was no hope .
That last comment really got to me. The vagaries of a GM can be explained away, you can turn down lethality by controlling encounters, but to give something approximating hope in the 40k universe is basically rewriting the underpinnings. The entire justification for the miniatures game, and most of the lore besides, is that the entire universe has been locked in a 10,000 year stalemate, with a slowly fading Emperor, daemons always clawing for a way out of the Warp, and only unwavering devotion to purity and faith keeping the Imperium safe.
There is no real hope. You're a soldier for a mix of the worst elements of the Nazi party and the Catholic church, and depending on who's writing, all that faith is your only salvation or absolutely meaningless. Any whiff of dissidence is met with purges and Exterminatus. Even the non-soldier population is restrained to a single world and a single task for their entire lives, living on one level of a hive city, working on an assembly line for war machines (if lucky enough for that!), the same job held by his father and his father before him. Even if you do nothing wrong, you may be purged because a heretic was discovered in your work group, or town, or company, and heresy is very broadly defined. The forecast for your life is endless toil with a high chance of early death.
The only way such a hopeless society is maintained is through the constant threat of retribution, the deaths of everyone who knows anything of the actual truth, and a short lifespan for all involved. The only reason constant unwavering discipline is so heavily enforced is because the alternative is the death of every living human in the galaxy, so who cares if we have to kill a thousand soldiers at a go because they learned that some Space Marines fell to chaos? It's a neat box to find yourself in, and it makes a lot of sense for a miniatures game based on never-ending conflict.
But I don't think it makes a lot of sense for an RPG.
If we ever do play, I think I'll borrow pretty heavily from World War II as my inspiration. The Siege of Stalingrad and the atrocities in the Pacific, the high casualty rates among the Airborne units that went in ahead of D-Day, the islands reduced to nothing but cratered coral as Marines and Japanese soldiers fought tooth and nail for a flat patch that could be used as a landing strip - these were all terribly tragic events, and often enforced with brutality, but there was still hope. The war wasn't endless. There was a better life at home. There were things to fight for that weren't a bolt to the back of the head. And I think that makes for a much more compelling RPG experience than the absolute darkness of the universe.
So my question to the group is - how do you handle the grim darkness? Do you help your players survive, or are they almost certain to die? Are they allowed to think for themselves, or does the commissar threaten any independent thought with death? Do you change the universe to make it a more compelling place to live, with a home life they might get back to someday, friends and family waiting for them, and a quiet, well-lit place for them to hang up their gear sometimes?
Or are you happy to fight in a galaxy where there is only war?