Since we are on about Mass effect, I believe that the replay value will be a big post-release seller so if it doesn't end up as a large game right away, you will see it sold out and hard to come by in the months that follow.
I still get goosebumps about Fallout 3 though. not only is it a wonderful series (butchered a bit by Sony with Fallout Tactics: Brotherhood of steel.) but it is post-apocolyptic chaos. And it sounds like it will make a cool first-person RPG kinda like Oblivion, although I doubt it will have the in-depth world of oblivion, I do have high hopes that it will be an action packed crazy ride from start to finish. And maybe they will have a larger variety of enemies with a wider array of skill sets to keep us guess and always adapting.