Slightly old thread and double post.
Just thought I would add that I just got and beat Mothership Zeta and its pretty cool. Its definetly worth getting, the quest if fun and you can get a ton of cool items and weapons. There isnt much to do in the ship once you finish it though, but you can go back if you want to.
Overall, I'd say Mothership Zeta is worth getting, but I would say that the best DLC to get is Broken Steel. If you dont have any DLC for Fallout 3, deffinetly get Broken Steel first, as its the most useful.
Oh yeah, also thought I would brag about having all the acheivments for Fallout 3, including all DLCs.