Mass Effect 3 - Extended Cut Released on Xbox 360 and PC

Kasumi
One new about 1h long quest and a companion with two unique abilities (somewhat "overpowered") who stays with you till the end of the game and sometimes comments on the quests you´ve finished. She also has some unique comments during quests here and there. Unlike the vanilla party members, she doesn´t have any dialogue wheel convos on Normandy. The loot from the quest is probably the best submachine gun (otherwise the most lackluster weapon category) in the game. The DLC comes with some great visuals also.
I really like the vibe of the first part of the quest - it´s very James Bond-esque which makes it a refreshing addition -, however gameplay-wise it´s a heist for dummies since you barely have to figure out anything by yourself. The second part is an usual combat affair.
In ME3, the character gets a quite cool cameo during one of the quests.

Overlord
This one breaks the gameplay mold a bit because it includes a slightly open world-y vehicular section (in a hover tank) on a planet´s well visualized surface.
It also includes vehicular combat which largely sucks.
Otherwise it´s an about 2h long self contained quest with System Shock-esque vibes here and there (especially in the pretty cool final section), a suspenseful story, solid atmosphere and effective end.
Companions don´t comment anywhere during the quest and the gameplay can get at times repetitive.
In ME3, there are possible two quite cool, though brief, aftermath dialogue encounters.

Arrival
This one is crap.
2h of mostly bad storytelling, boring environments, repetitive gameplay, plus one annoyingly forced "choice".
As a solo mission it had potential to deliver a unique experience, but besides one rudimentary stealth section and one tough combat scenario (which, admittedly, is a fun way to test player´s character who for once can´t patch his/her weaknesses via squad mates), the DLC doesn´t take any advantage of it and the encounter design is uninspired.
The quest becomes available around 1/3 into the game and it´s presented as something really urgent so leaving it to ME2´s post-end doesn´t feel right, but if you do it early the rest of ME2 might feel inconsequential and if you leave it till post-end (or install it only by then) you´ll end your playthrough on low note.
Besides two conversations at the very end and the aforementioned one combat scenario, it´s mostly terrible, really.
The good thing is that as a bridge between ME2 and ME3 the DLC is quite dispensable - the final ME2´s cinematic is enough of a bridge when it comes to the core threat and if you haven´t played the DLC, ME3 does treat it as canon, but only as something that happened, not as something Shepard was involved in. You have to fill one blank in ME3´s beginning for yourself, but well, ME3´s beginning is abysmal anyway .
I probably wouldn´t recommend this DLC even if it was free since I think it makes ME2 worse.

Lair of the Shadow Broker
This one´s the best of the bunch.
In ME3, what happens in this DLC is considered canon and as in the case of Arrival, if you haven´t played through it, Shepard is not considered to have been involved in it.
This DLC however pertains to major character development of one of your squad mates and since ME3 doesn´t take much time explaining it, not having played through this one is a lot more noticeable than in the case of Arrival.
The good thing is this is a really good piece of content.
It involves one of the companions from ME1 (and ME3) and a mysterious figure introduced also in the first game. The story development in this DLC is good, though not particularly stellar, gameplay feels fairly varied because a) there´s a simple, but fun car chase section, b) one of the set pieces involves superb visuals, c) in the middle of the quest there´s a really cool boss fight (which turns into an amazing one if you happen to play a proper vanguard, proper, as in one who doesn´t hesitate to charge + shotgun) and there´s a good chunk of content once the main quest is over (it´s mostly just watching short videos and a lot of reading, but it fleshes out a lot of characters quite nicely).
I´d say it takes about 2h to finish + up to 1h for the "aftermath material".
Just as all the other DLCs, I´d say this one is also somewhat questionably priced, but if you happen to enjoy the game at least to some extent, I´d say this one´s a must have.

In terms of insertion into the game, Kasumi DLC is better obtained early since she´s an additional companion, Overlord and Shadow Broker imo work best when played somewhere in the game´s third quarter (roughly the point when the main quest starts to "wait for your approval" to continue - by that time the galaxy map is fully opened and you can "free roam" at your leisure and finish as many side quests as you want), the very last Shadow Broker conversation (on Normandy) is better to be postponed till after the endgame for the best effect.
Arrival is best not inserted at all.