Currently going through the FO3 DLC right now. Finished The Pitt, Operation: Anchorage, and Mothership Zeta, and currently going through Point Lookout.
I thought The Pitt was ok, and the moral dilemma was interesting (the only time I ever thought my character could side with the raiders), but I dunno, I thought it was merely middling.
I like Operation: Anchorage a lot, but it's basically an early modern shooter dropped into the game, so I think it's dependent on if you like those or not (confession, I enjoy them).
I liked how Mothership Zeta felt, and the new look, and the weapons, but the levels could be a bit confusing, and, when you're in there it's cool, but once you're out it feels... weird, almost jarring. Also, you should be able to see the destroyed wreck in the sky when you look up.
Point Lookout so far is... eh. I like the idea, and the look, but so far it's not really enthralling me. I would love to play a Fallout game set in Louisiana, but honestly, I'm preferring the DLC and main game of New Vegas a lot more. Bethesda should just hire Obsidian for every Fallout game, I think.