First I'll tell what I didn't like about it, it should have been released with the game! Not that I wouldn't have mind to pay for it, but I would have liked to play this awesome DLC along with the main storyline on my first play-through. I guess I will see how this DLC feels when I do a replay.
Now what I do like about it: everything! If only this was the real ending to mass effect, but I guess this being the final DLC this is the ending for most players.
The story line is about an unknown group trying to get rid of you. You will annihilate this group with all your friends, and a lot of jokes and remarks keep you going through this story line.
You will also find out something shocking about the Genophage.
Because of all your friends there, it feels a bit like the suicide mission but without the difficult choices and seriousness.
After the (short) story, you can enjoy some moments with your current friends and some from the past. I imagine playing the last real mission (attack on Cerberus) and thus ending the game after this dlc must be even more emotional.
Now, the universe we all know has come to an end, and there is only hope left.
Hope that Bioware (and... ea) do the ME universe justice with ME4.