Borderlands satisfies both RPG and FPS craving in one shot.
You start by choosing one of 4 characters. I chose the rifle dude and headed out. Comparing this game to Fallout is not a safe way of approaching it. It's a complete experience in it's own. Multiple missions and weapons combining makes for a greatly varied experience.
I haven't played for too long (few hours) but I will say that I'm impressed with the solid feel of Borderlands so far. Would highly recommend that you play this game, especially if you are an FPS or semi-RPG fan.