If you're playing for story, don't. It's like Mass Effect meets gears of war minus the blood and guts.
As far as the Citadel DLC goes, the SP portion is a 4GB download. On Xbox Live, DLC can only be 2GB so they've split into two seperate downloads. In other words, it's going to be huge.
My take on it is that it takes place post-ending. Why? Because the in the description it talks about uncovering the truth (which we're still waiting on because there was nothing in the vanilla ending) and that after, we can relax with our squadmates, have drinks, gamble, and continue relationships...oh, and we can purchase/furnish an apartment on the Citadel. In other words, Mass Effect: The Sims.
Now, tell me if I'm wrong, but I just can't see this taking place during the main storyline. If so, it would sound like this:
Anderson: "Shepard! We're getting hammered down here! Is the crucible prepared?"
Shepard: "Yeah we're good to go Admiral, but before we launch the assault to take back Earth and end this war I'm going to send you a picture and I need your feedback...see, I can't decide if I want this couch pressed up against the wall or if I should set it by the window over there.... Also, do you think the leather is too much or should I send it back and order fabric? Oh, hold that thought! Drinks have arrived! Later bud, hold that line!"