Made by the company behind the fantastic Baldur's Gate role-playing games, Mass Effect is the first sci-fi RPG I've ever played, and it's fantastic.
The main plot line involves a baddie named Saren who wants to wipe out humanity (and everything else) by finding and unleashing a race of intelligent machines called the Reapers. The scope is monumental, the look sleek and futuristic and the soundtrack well composed -- all things you would expect from a game of this type.
However the little things really make this game stand out. There are many subtle nods to actual science such as naming a location after Stephen Hawking and a type of ammo made up of Bose-Einstein condensate. The alien races are truly alien - both in look and behavior. The beefy elcor preface each sentence to non-Elcor with the emotion that is meant to be conveyed because truly understanding their native language requires the listener to be in tune with subtle scent and body posture variations. Are these sorts of details really needed? Of course not, but it certainly helps the immersive feel of the game.
There's lots of action, lots of sleuthing and lots of fun to be had. My verdict: this is a game well worth buying instead of renting, especially if you enjoy a good RPG... or alien ladies in tight-fitting body armor.