Although my original thought was to tackle the Fallout 3 add-ons in the order they were released, I decided on a whim to jump ahead to Mothership Zeta (add-on) after acquiring and installing it a bit after I had already started my post-main-quest adventures. I actually played through MZ a few weeks ago (just after finishing Operation: Anchorage), but I got distracted from writing this post.
Interesting to note is that the DLC starts at the Alien crash site from the core/pre-addon version of the game, which I had already found during my earlier adventures. Returning to that site with Fawkes and Dogmeat, I was sucked up into the Mothership while my companions stared on helplessly (and waited in that spot for my return). It was a bit disappointing that - as with Operation: Anchorage - my companions were not able to come along with me on this DLC, but Bethedsa made up for it with some new temporary companions that I will discuss in a bit.
After arriving on Mothership Zeta, I immediately noticed that the architecture was completely different from that of the rest of the game. Even the simulation of Operation: Anchorage had some familiar architecture, equipment, character types, etc., but the vast majority of this add-on had the appearance of being almost completely original. This really gave the impression that Bethesda put a lot of work into the last of the Fallout 3 add-ons.
I found especially enjoyable the substantial cast of friendly NPCs, some of whom appeared right at the beginning of the add-on and stuck around all the way until the end. Some served to guide my progression through the add-on's quests, while others served as temporary companions that teamed up with me to explore the various side areas. While their back-stories were all rather lacking, they were all unique (coming from different times and places throughout human history) in appearance and personality.
There was of course a plethora of new equipment as well; most notable was probably Alien epoxy, which can be used to directly repair equipment without the need for parts from other weapons/armor (although its effective non-repleneshability will likely discourage me from using it). Most of the guns were energy weapons, although they did provide pistol, rifle and big gun varieties of them (with unique variants of each), and there was also a new melee weapon or two (again, with unique variants). As with most new items I've found in the DLCs so far, all of these went into my weapon/armor locker in My Megaton house; in this case, however, it was mainly because I knew I'd effectively run out of alien ammo eventually. No sense in changing tactics and getting dependent on equipment that I won't be able to use indefinitely!
I felt that this add-on did drop the ball a bit when it came to enemies, unfortunately. Not counting recycled enemies used as cannon fodder, there were I think no more than a half-dozen or so new, non-unique enemy types: 2 hostile alien types (helmet and no helmet), 1 semi-hostile alien type (red on compass but always flees and costs karma to kill), 2 robot drone types and Abominations. This made combat in the add-on feel rather repetitive at times, with enemies sometimes almost becoming little more than distractions that were getting in the way of exploring the rest of the content. Oh well; at least they dropped enough guns and ammo that I felt like I could use that looted equipment while on MZ, and still have plenty left over to squirrel away in my shack after all was said and done.
A bit of the beginning and end of the add-on content was linear, but the middle portion put me in a hub area connected to multiple other areas that could be tackled in almost any order. These areas formed the real meat of the add-on content in terms of time spent and ground covered, if not in terms of plot, and each was more than unique enough to prevent my exploration from feeling at all repetitive. The hub itself was also made into more than just a boring pit-stop/crossroads by a gradual accumulation of junk architecture and various items, presumably made present by the scavenging efforts of some of the friendly NPC's while I was off exploring each of the side-areas.
There wasn't much story to speak of in the add-on. Bethesda passed up the opportunity to really ground it to the rest of the game universe by neither fleshing out back-stories for the friendly NPCs, nor completely exposing the mysteries/motivations/goals of the aliens. In the end it was really just about exploring, killing and looting everything with the ultimate goal of everyone being able to return to Earth. Fortunately the characters and level design/flow/architecture kept things interesting enough to distract me from the lack of plot. There were also logs that the aliens forced their captives to record, which offered a bit of entertainment and flavor, but unfortunately did not shed much light on the aliens' motives or back-story.
One big problem I had with this add-on was that, as a result of being able to bring gear along with me and bring loot back to the Capital Wasteland after I was done, I found way, way more loot than I could have possibly hoped to carry back to my Megaton shack in anything close to a practical manner. At first, I started stashing things on dead bodies at the start of each area, then clearing out said area and going back to haul the pile forward to the next area. Eventually this became too tedious, and I employed console cheats - warping to a specific body at first, then finally modding my maximum carrying weight to over 5000 WG. My justification (to help me sleep at night) was that I could have managed to keep all the loot by my original, tedious method, so all I was doing was avoiding a needless, boring waste of time that would have otherwise negatively impacted my enjoyment of the add-on.
There were also a few bugs and annoyances. Probably the worst was a well-documented issue with a popup message appearing over and over when I got to a certain point in the add-on (which turned out to apparently be related to having a robot in tow); fortunately I figured out a workaround for that one before I even looked it up. There were also a couple of geometry/texture glitches, but nothing too horrible.
In summary, I would say that this add-on is worth checking out if you're interested in getting good equipment and getting a break from the regular Fallout 3 game feel; if you're looking for story, though, you'll probably be disappointed.