In my opinion the recently released Citadel DLC was the best DLC in the entire Mass Effect Trilogy. It was as polished and as fun as Mass Effect 2's Lair of the Shadow Broker, which is often seen as the best DLC in the trilogy. "Citadel" was loads better because instead of focusing on one character, it focused on all of them, and in my opinion the characters have always been the focal point of the entire series.
The new hub area (the apartment and strip) are great to walk around, but I did feel that the strip was maybe missing some more shops to waste money at, but there is loads of quasar to use up my credits. The ability to meet up with squadmates in your apartment and around the strip is awesome and I like the fact that they automatically unlock as you progress through the main game.
The party that Shep hosts later in the DLC is hilarious and allows you to meet up with all your former colleagues from previous Mass Effect games (except the ones who are dead/in hiding etc). The party can either be a rowdy one where everyone gets drunk and "blows of some steam" (which was hilarious) or a more relaxed affair with food and drinks. This part of the DLC is ripe for re-playability as there are many parts that you will miss on one play through. I also suggest that you try it at least once with a non-romanced Shepard for hilarious consequences. The very final scene of the DLC is guaranteed to make you tear up a little and is a beautiful way to say goodbye to series.
Overall the DLC took me about 6-7 hours to complete and even then I did not see every possible outcome from it or spend much time in the combat arena. For only 1200MSP this is a great swan song and a must buy for any Mass Effect fan.