OK, I am late to the party, but I didn't find time to play it earlier than last week. EDIT: I also only red to page 9 (forum default) of this thread. Sorry if stuff has already been mentioned.
The new content is really amazing and as a whole it is an awesome achievement.
I have a few issues with the mod, though.
Please note that the follwing statements aren't saying that the mod is bad, but what (IMHO, of course) prevents it from being something really outstanding not only within the various DN3d-mods but also outside the DN3D community.
I personally find the story and the introduction of new enemies and Bombshell a bit lacking (talking Episode 5 of version 1.02). Both story and gameplay should go hand in hand and give the player more time to see, admire and learn about the enemies (yes, I am aware that the first map started out as DM map). And that needs to be within a story that suits the gameplay and the progression through the levels.
Bombshell is just there (in the intro and suddenly at a mid-bossfight). IMHO it had been better if there was some sort of story around her like that the EDF is messaging Duke about a missing agent that was investigating in the red light district of the city ... and then you rescue Bombshell who is then your NPC partner. Of course some of the strippers there were nothing but agents for another nation that now work together with the aliens and give them information. On of the aliens' first targets was the bath house that is now surrounded by aliens and Duke needs to get in and rescue the babes there (the opening scenario bath is a highlight when it comes to new content and should be featured later and within the story as a target of interest and a plot point, IMHO).
Tease the player with what will be happening, then suprise him with awesome results and let it all come together in the end.
I also don't like the precision of the railgunner aliens. If you use the railgun on far away railgun aliens yourself you have to zoom in which takes time. THe aliens on the other hand aim perfectly much too fast. In a duel they have almost perfect aim instantly while you still need to zoom in and adjust your aim. If they needed longer to aim that good and loose ther aim again when you move it would have added another layer to DN3D gameplay (again, IMHO). As it is now it is standard DN3D gameplay but with a railgun. Look, you can even use a sniper in Serious Sam and it doesn't subtract anything from the carnage parts of Serious Sam.
Here in Alien Armageddon you encounter the first Railgunner in close quarters and in a bathtub. Then they are suddenly somewhere on the roofs at several places at once firing at you. It had been better to have one scene in the level where visually not too much is happening and it is quite clear that there is one of these Railguunners firing at you from a position you can clearly see. Later the player knows that they tend to stand around on rooftops and fire at you. And don't forget to incorporate them into the story.
Conclusion: There is so much awesome stuff in the mod that it makes me a bit sad that the true potential on how to present that content and new gameplay isn't fully used.