The easiest way to grasp Mass Effect is to imagine Knights of the Old Republic. Strip out the Star Wars licence and replace it with some serious-but-well-crafted original fiction, then remove all the turn-based combat. Replace it with competent but unspectacular third-person shooter action, complete with cover system. The RPG skills influence the game, in terms of special powers and improved shooting abilities. So, like Deus Ex, the amount your sniper-sight wobbles is based upon your character's ability and the quality of your weapon. And, like Deus Ex, its worth is less in the individual action sequences, and more in the synergy between the RPG development, the action and the frame of a developing story. The results are splendid. Kristan gave it a strong eight, and that's what I'd give it too, so if you're one of those sort of people you can go and get back to comparing PS3 and Xbox 360 screenshots for errant pixels.
If you have the Xbox 360 version of Mass Effect, you're in a pickle. On the one hand, there's nothing truly new here. On the other hand, playing the game on a mouse and keyboard is like playing a brand-new game. Unlike its console predecessor, the PC version makes combat exciting and fun, depending less on auto aim and more on real skill.
After two playthroughs on the 360 and a third on the PC, I can honestly say that this is how the game was meant to be played, and the experience is incredibly refreshing. If you've never played the game before, then this is an unequivocal must-buy. If you have, and the money's knocking about - then you still might be interested in playing the definitive Mass Effect.