Kasumi or Overlord?

Kasumi was an hour of short and fun.  Overlord was a half hour of fun and that was at the end, most of it just drags.

Kasumi offers a squad member who's surprisingly competent in combat since she can cloak and attack from behind.
Overlord is a bit longer and maybe offers more in story and intrigue but on the other hand, there's nothing that makes a permanent mark on your playthrough.
 
I would go with Kasumi.

I personally think that Overlord was better than Shadow Broker. The variety in the gameplay is pretty good, although the pacing can drag a bit. 

Overlord is probably better on it's own, but if you're going to be playing the rest of the game along with the DLC, then I would recommend Kasumi since you get the squad member.

Kasumi is a one area mission that nets you a new squad member along with some weapons and other goodies. Overlord on the other hand is a rather lengthy series of missions but in the end doesn't leave you with anything permanent. They're both good but you have to choose which you're looking for. 
 
Or you could just get both.

What do you want from your DLC? If you want something akin to shadow broker in it's structure then get Overlord, if you want a new character and one new loyalty mission get Kasumi.
 
Be forewarned though that Kasumi is like Zaeed in conversational nature in that you can't have actual dialogue with him he just talks at you.

Neither is especially great, but Kasumi at least nets you another party member as well as a surprisingly useful SMG. Overlord was kind of.... bleh.

The only really good DLC is Shadow Broker. And as much as I hate the Hammerhead portions of Overlord, I hated even more how stupid Kasumi's mission was. The game goes out of its way to give Shepard an elegant suit then acts all surprised when Kasumi can't get inside when she's dressed like a freakin assassin!!
 
Anyways, neither is great, but in my opinion the story and especially the ending of Overlord makes up for the rest.

None really. The only real worth while dlc is the shadow broker. Save your money. 

Well, as far as Mass Effect 2 DLC goes, it's Shadow Broker, then Kasumi, then Overlord. Overlord is longer then Kasumi, but I felt that Kasumi was more fun.

I'd get Kasumi just because she becomes a party member that's pretty useful.
Overlord was really good though - it drags some in the middle parts but the last half hour when there's a lot of plot stuff it gets fucking brilliant. One of the more intense mission plots, definitely.

You should have bought all of them when they were 200 ms points during the christmas sale, that was pretty awesome
 
Anyway I recommend Kasumi because it's alot better and Kasumi is a great character, and really useful during missions

I don't really see why people rave about the Shadow Broker. The end was good but the rest was pretty uninspired. Overlord was better start to finish. Kasumi was great but as mentioned before it's pretty short. 

If you have to choose one, choose overlord, but ideally getting both of them is preferable.

Kasumi is just okay, but gives you a very useful character.  Overlord is great the whole way through, but you don't really get anything out of it.