My regular Melbourne roleplaying group started playing the Warhammer Fantasy games a longwhile back, and our current GM is somebody who loves to try new systems, especially those which hook into our other interestes, and have great lore attached.
Thus I came across DeathWatch by Fantasy Flight Games.
Here is a quick review of the DeathWatch RPG game based upon the games played so far.
It is a highly addictive and compelling game model, that removes the need for detailed backstory and allows a drop in, start shooting approach. This is excellent, and something that I would not have though would work had it been suggested to me. Plain honest fun.
The core lore (sometimes called fluff by the tabletop gamers) is so darn deep it is almost impossible to be an expert on any of it, yet somehow as a player you adjust and enjoy. The game lore itself has been altered over the years, and it is an ever expanding base of content, driven by the tabletop players (when they do retcon lore adjustments which are still plausible, but the game themes themselves allow for it is just a wonder circular loop).
In short: play it. Its excellent.