Just finished the Citadel DLC. Let me answer the one question everyone wants to know, is it worth it?
The short answer, YES!
All of the Normany crew is reunited for a clever plot that contains tons and tons of fan requests integrated into the story. I don't want to spoil the plot for you, but the writing is pretty good. It is mostly fan service, yes, but it is really good fan service.
Most of the things you'd look forward to are here. Wrex is back and available as a squadmate. The environments are absolutely stunning. The screen is chalked full of eye candy throughout the majority of the DLC. New enemies, a great boss battle, several legs of the mission, it's full and satisfying. The central hub added with this DLC is customizable and contains several letters from Captain Anderson, which who wouldn't want laying around their pad? Tons of dialogue was written for this and I found myself lingering just to talk to every NPC I came across. References to the N7 Fury (added for multiplayer), and other little tidbits of lore are spread throughout. And the main script has nice (subtle) jokes that long time fans of the series and members of the forums will recognize. Things like Vega saying, "Why does no one ever pick me?" had me chuckling out loud several times during the mission.
And after the main mission, there is still more content. I'll tease just one. Blasto. That is all.