Definitely a keeper...
I never thought Mass Effect was a good game in general. A guy flying through space in tights with a gun shooting aliens. But really its a lot more than that, the ability to take cover during a firefight, make up your own story with different characters, and.. there just seems to be so much freedom in the game that I actually like it. Bioware really outdid themselves when they started allowing people to choose how they wanted to talk to people or choose what to do during the game, it really changes how everything plays out and feels.
The main thing I want to express to you if you haven't played this game yet is the choosing. Graphics are great, combat is great, guns are amazing, special powers are dead on, everything works but the main thing about the game is the choosing. Throughout the game there are times when the screen starts to flash allowing you to choose what you would like to do during this time, so (taking this straight from the game) your saving someone and must hold out in a small fort with him by your side. He scouts out the enemy with his sniper rifle, and lets you take a look. Across the bridge comes a wave of men - with your scope targeted you can either take a hostile action and gain some evil points by being a coward and shooting one of the guys off guard or you can just leave them be and gain happy points (but you only have about 3 seconds to make up your mind). There are many instances like this during the game, and choosing one of those actions could change the whole game from that point.
If you haven't played Mass Effect 2 definitely give it a shout. Its going to suck for the first hour, which RPG doesn't suck for the first hour? But later on in the game as you start to get out into the world and immerse yourself you will notice you are addicted and attached - you must continue playing until you see the final result.
Good Luck!