I'm a bit late to the party, but I'm mid way through Mass Effect, and Wow. A game where the characters have distinct believable personalities, a varied and interesting game world and, cracking gameplay with solid gunplay. Despite the sci-fi 101 storyline (so far anyway) I'm enthralled. Just in case there are any out there who have yet to play:
The choices are difficult because of the emotional attatchment to the characters, having to decide who to save (and who to sleep with) actually feels integral to the world and plot, rather than a tacked on gimmick.
Absolutely love it, I may even consider paying for the extra downloadable content - something I've never done.