I just finished it and I have some veeeery good news :)
The length is just ok... it is made from one small and one huge area to fight in,it took me 1,5 - 2 hours approximately. A few nice fights, a great last stand and a few twists (the whole mission is solo and there's even a small stealth segment).
Also, the ending is just brutal, I wonder what will they do to Shepard after THIS :) Just wow.
Negatives... The story is a bit illogical at times, but nothing too bad. I'd give it 8/10. A great teaser for Mass Effect 3, I wanna know what happens now :)
serpantino: I hate it when they do things like this. Release DLC that you have to buy to be up to speed for the inevitable sequel. I completed and uninstalled Mass Effect 2 a long time ago. (Thoroughly enjoyed it.) but this should either have a standalone story line that doesn't continue the main plot or be part of the 3rd game; damn greedy Bioware.
StingingVelvet: I doubt it will actually be that mandatory. They acted like Witch Hunt lead in to DA2 but I would say it just barely did and was completely irrelevant to the greater story.
This one is mandatory, unfortunately. It's a pretty big intro to what happens in the next game.