Citadel is a fun add-on that is also oddly placed and irrelevant to the Mass Effect 3 storyline.
Cons: Has no relevance to the main story, Feels totally out of place, Some story elements are predictable
Citadel marks the endpoint for Mass Effect 3 DLC. In a way, this marks the definitive end to the Mass Effect trilogy as a whole. This expansion allows you to reconnect with old squadmates, rekindle your romance with lesser-seen characters(Miranda), and explore new, beautiful areas. It is a light-hearted add-on that is packed with humor and fan service.
The story will take you through many different areas of the Citadel that you've never seen before. Each area is intricately detailed and well designed, sporting tons of lighting effects. I'd go so far as to say that Citadel is the best looking of all the Mass Effect 3 DLC.
Despite this, Citadel's place in the Mass Effect 3 story is largely irrelevant. It is optional to start with, yes, but this doesn't provide any missions that play a big part in the story. Lair of the Shadow Broker played a significant role in Mass Effect 2, while still remaining optional. Citadel, by comparison, feels out of place.
Citadel is supposed to be a light-hearted adventure, and it does achieve this, but it contrasts heavily with the rest of Mass Effect 3. Shepard once says "I'll sleep when I'm dead.", yet this expansion allows you to party while Thessia burns and the Reapers occupy Earth. It just doesn't make much sense.
This DLC is meant to allow players to reconnect with old squadmates in one, last get together before retiring Shepard. It could have been done in a way that was more relevant to the story, but Citadel provides great fan service nonetheless. There is plenty of funny moments and enjoyment in this DLC, so I don't recommend skipping it. It may not be everyone's ideal way of closing the door on this trilogy, but it's not terrible either.