Arrival is the seventh and presumably final mission-based DLC for Mass Effect 2.  In it, you learn that an Alliance deep-cover operative has been arrested by the batarians for being a terrorist, but that she might have evidence of an imminent Reaper invasion.  That means you have to rescue the operative from a secret prison and then learn what she knows about the Reapers.  Of course, almost nothing goes smoothly, which means you also have to shoot a lot of stuff.

Unfortunately, the quality of Arrival is uneven.  You have to complete most of the mission solo, which adds some difficulty to the proceedings, and there are a couple of interesting sequences -- including holding off a barrage of attacks, and "stealthing" your way through the prison complex (both of which give you achievements) -- but otherwise there isn't anything in the DLC that you haven't seen before.  I mean, the closest thing to a boss is a YMIR mech, which stopped being an exciting enemy about halfway through the OC.

Arrival is also a bit on the thin side.  Despite only taking a couple of hours to complete, many of the locations appear to have been added just to pad the playing time.  You end the mission at a base on an asteroid, and somehow this base has something like eight elevators.  It's like somebody at BioWare kept checking in to see if they had reached two hours yet, and when he found they hadn't, added in another floor and another minor fight.  Worse, the DLC is completely linear, none of your choices change anything, and the premise itself is a little bit strange if you've already completed the OC.  I mean, at that point you don't really need any extra proof of an invasion.

The Arrival DLC costs the same amount as the Kasumi and Overlord packs, and less than the Shadow Broker pack, but it is significantly less impressive -- especially since it feels more like an advertisement for Mass Effect 3 (which is currently due out before Christmas) than it does anything else.  But Arrival has a few fun moments, and it includes a few new upgrades for your character, and so it's not a complete waste of time.  So as long as spending $7 on a two-hour mission doesn't bother you any, you should feel free to pick it up.