GAME OVER MAN!!
While not the best of the DLC, Mothership Zeta was a fun experience that made it worth the money. While wandering the wasteland you pick up a radio signal that takes you to the fallen Alien Ship. Once you arrive you get beamed up, put in an observation chamber and then the adventure begins. It is all about freeing yourself and some new friends at first, until a small (but obvious) twist shows up.
Similar to Operation Anchorage, Mothership Zeta is a pretty linear experience. The game offers you "freedom" by letting you choose the order in which you complete 3 tasks, but everything else happens in a straight line. But that is fine. I enjoyed Operation Anchorage and I enjoyed Mothership Zeta.
I feel like each DLC has a mechanic that they try to work with. Anchorage had a small section of squad control. Point Lookout had the most open world experience, and Mothership Zeta allows you to grab followers as you enter each of these three zones. While you have a number of friends that are willing to pitch in, each one will only go in one section with you. I didn't really matter though since I spent most of my time stealthily going through (repetitive) hallways so I left them behind.
Where Mothership Zeta held its charm is in the fun story, a handful of beautiful scenes, interactions with Stacy and plenty of combat. The bonuses you get are pretty slim. There is a handful of new weapons, including a unique version of an Alien Atomizer. You get a spacesuit, and tons of "Alien Crystals," but all of that feels pretty stale. Perhaps the best feature is Somah, who has the ability to completely repair all of your equipment. I used this repeatedly as I went through all of the combat, and I paid for it in the form of 4000+ credits over the course of the whole DLC.
I can easily see people putting Mothership Zeta at the bottom of the DLC pile for Fallout 3. While the game experience was fun, it wasn't the open experience of Point Lookout, or the continuation of the story like Broken Steel. It's most direct comparison is Operation Anchorage, but at the end of Anchorage you have most likely picked up the Chinese Stealth Suit, which is much more useful than the alien weaponry. Add to that the fact that you are likely to be at full level by the time you hit the Mothership and you have a DLC that Fallout fans will enjoy because it is more Fallout, but more casual Fallout players may be disappointed in.