Mass Effect 3 has had a hard time over the last year since it launched. There was outcry about the From Ashes DLC being for-pay at launch, and the endings took a lot of heat. The endings got tweaked with a piece of free DLC, Leviathan was received well, and Omega was something of a letdown. Citadel is a swansong for the trilogy, and a glorious one it is.
The combat portion is about two hours worth of content, and it is brisk and fun to play. That alone would be a good chunk of content, but that's not where this ends. Instead, you get to meet up with all of your living comrades on the Citadel. In fact, you even get to throw one hell of a party. It's a beautiful send-off to a great trilogy.
If Lair of the Shadow Broker was the high bar for DLC before, Citadel has surpassed it. This has everything you love about Mass Effect crammed into about four hours of content. If you've played through Mass Effects 1-3, you need to spend the $15 on Citadel.