OK, I just finished it about an hour ago. This year has been a monumental one for video games. Bioshock, Super Mario Galaxy, and now Mass Effect have all been released within the last 3 months and they're all some of my favorite games of all time.
I think I'm going to open another thread for spoiler-filled plot discussion, but I have to say that I've yet to see a world of fiction so in-depth and well realized in a game. Also, it seems like Mass Effect achieves what KOTOR was aiming to do but couldn't quite achieve: making you feel like your choices really do matter, and forcing you to make really hard decisions. I think removing the whole light/dark thing and replacing it with the more ambiguous system helped that to an extent.
Also, the dialog system is totally brilliant. Sure, the conversations precede similarly in some cases no matter what you choose, but it feels like you're so much more in control. I really felt like I was in control of Shepard, not just trotting him along some path that had been set out beforehand. It's a subtle thing that makes a world of difference.
The game isn't perfect by any standards. All the criticisms about load times and texture popping and framerate are totally legitimate, but I can't remember the last time I've enjoyed a game this much. This almost rivals BGII, in my opinion. I can't wait for the next one. I'm moving on to Assassin's Creed, but I fully intend to go back and do a second playthrough with my badass biotic renegade.