DX is one of my all time favorites. The role playing is fairly shallow, but you do get to make your own decisions and those decisions do change how other parts of the game are played. A good tip for veterans and noobies to the game, is read everything you find and talk to everyone to get everything out of them that you can, even the people that just say stuff and you dont actually go into conversation, you learn a whole lot more about the atmosphere and plot that way... god, i CAN'T WAIT for Invisible War(DX2)...
and yeah, if you've played System Shock1/2, its the same kind of game. the atomosphere in SS is amazing, but DXs plot is way crazy good, IMO. conspiracy is just my thing i guess.
oh yeah, one more thing... DEFFINENTLY not a Diablo clone... totally different kind of game.