I prefer the Abnett-verse for my 40k RPG needs, but if I play the wargame, my inclination is towards the ridiculous gonzo early days - both the 2nd Edition (same basic premise as the current setting, but sillier and more 2000 AD showing through), and Rogue Trader (no Chaos explicitly mentioned, half wargame/half RPG, Space Marines are crazy psychos instead of boring warrior-monks).
While checking around for resources to run a Dark Heresy game over G+ using a system that's not the FFG Dark Heresy ruleset, I happened across this fellow, who's blogging about an OSR movement in 40K. I'm absurdly happy about more folks being interested in what I find to be some of the coolest stuff in 40K.
(Oh, and ol' Mandamus gets a shout-out there too!)
And now I'd better get back to this final paper. Last one for law school...!