I actually really liked Shadow Broker. Kinda neutral on Overlord, it was okay. The Kasumi one was just not very good at all. The biggest problem with these things is that they're so short, and don't really give you a bang for your buck.
Biggest disappointment of the Kasumi DLC: they never let you identify yourself as "Shepard...Commander Shepard."
Actually, I would have loved it if it were longer. Problems I have to espionage my way through instead of the classic BSU solution? Why, thank you.