I swore I would never touch another piece of Mass Effect content after 3 kicked me in the nuts.
I am very pleased to say that ME3: Citadel is probably one of the best piece of Mass Effect content to date. It's better than ALL of ME3. It has some amazing story moments, fleshes out key ME backstory, has plenty of great character moments, it's FUNNY and SELF AWARE and DOESN'T TAKE ITSELF SUPER SERIOUSLY.
The DLC took me maybe around 4 or 5 hours total to complete. The first 3rd is combat heavy and goes through a plot to kill Shepard while he's on shore leave on the Citadel. The plot is so good in this section. It's tightly written to span an hour to hour and a half chunk of time and that works far better than anything in ME3. After you finish that you spend the rest of the time walking around new parts of the Citadel and hanging around with characters from all 3 games. You pretty much get 1 or 2 or sometimes 3 specialized character vignettes... depending on who you have alive. My favorite aspect of ME was always talking with my squad mates, just hearing what they have to say. Citadel does this damned well. You get some really great scenes out of these.
Citadel also manages to take the RENEGADE/PARAGON system and do something legitimately cool. Well, cool may not be the right word but it's different.
If you want some FUN Mass Effect content that isn't UP IT'S OWN ASS and USES IT'S CHARACTERS correctly, BUY THIS.
It's FANTASTIC.