Mothership Zeta is a terrible DLC that is both dreadfully boring and utterly baffling.
Before I proceed to talk about the game I must issue a spoiler alert. But if I were you, I'd much rather hear about the plot now than finding it out myself after around 2 hours of tedious game play. The plot is insultingly simple: you get on the alien spaceship, and then you get off. The end. Along the way you may ask, "well, why am I being abducted by these aliens? What were they going to do to me? Why are they here?" Unfortunately these questions will not be answered. There are no instances where you can facilitate any sort of communications with the aliens for you to find out about their motives, nor are there any human peers that can fully understand what is going on. Although there are a number of audio recordings that document the interactions between the aliens and various other human abductees, they generally raise more questions than answers. To sum it up, playing Mothership Zeta is like watching a Lost episode: you walk away more confused than you were before watching.
To the developers' credit, there are some points in the game where it starts to show promise, like when you encounter other human abductees that are from another time. Nevertheless, the plot doesn't allow you to interact with them all that much, and again you will never know why they are even there.
A lack of creativity in game design will also strike you very early onwhile playing. The aliens are your stereotypical aliens: frail, green, large-headed, and big-eyed dwarfs. They are the majority of the enemies that you will encounter, and they come with very little varieties: the aliens who wear a helmet, and aliens who don't. Occasionally you will encounter assistant droids and human-alien fusion called Abominations (whose presence on the spaceship are also unexplained), but they don't come out very often. It is astounding how little creativity was put into designing this DLC, this doesn't only apply to character designs.
The level design of Mothership Zeta can be described by one word: lazy. It is very obvious that some of the maps are cut and pasted; everything simply just looks the same. Throughout the game you only do one kind of puzzle to proceed: overheat the generators by messing with their coolants. You do this for about five to six times in the game and after the sixth time you are pretty much at the end. The whole game is unrefined and boring, unlike its predecessors such as Point Lookout.
Another aspect of the game that's highly anticipated yet highly disappointing is the new weaponry. There are alien pistols, alien rifles, alien batons, and a rail-gun like cannon you can obtain from droids (incredibly useless). That's it. There are specialized versions of these weapons lying somewhere in the game and they are relatively powerful, but the fact that this is the very last DLC of the game makes it pointless to look for them.
In short, Mothership Zeta is a terrible DLC that is both dreadfully boring and utterly baffling.