Not worth it if you've already finished the game
As somebody who had already finished the game, the value of the add-on was extremely disappointing.
It starts with a pointless (read: time lengthening) visit to the citadel to have a minute long conversation before departing to the real destination.
The mission (SPOILER) is broken into two parts. A silly series of "fetch" quests that provide no difficulty (the problem with these types of quests is that they are impossible to fail and the size of the level always works against the thrill of finding the item...i.e. there are only a half dozen rooms to look in)
The second part is a running battle followed by a classically silly "boss" fight with the way overused cliche of the regenerating shields...in other words, you are going to play through the 3ish stages of wave/boss/wave/boss/wave/boss no matter what.
If you've already finished the game there is nothing else to do once Kasumi joins your crew. Frankly, with a team of like ten and only 2 joining on every mission, I think it's unlikely that most people would choose Kasumi as a mission regular.
Had it been free, this would have be a 7.5...it was a reasonable hour long mission...at $10, it's way overpriced and doesn't provide nearly enough value to be worth buying.