Their range is ~incredibly~ small. Maybe I am missing something basic in how you deploy them, but they have a high energy cost, a tiny unit cap, and less than half the range of tractor beam turrets. This basically means you need two per wormhole you want to protect from astro trains (one for the turrets and one for the tractor beams), possibly even three if you want to have a minefield of a decent size.
I only ever use them to protect my cluster of tractor and tachyon beams (100% the most important defensive structures in the game imo), as for the turrets they draw the short end of the stick for me. Generally I've got my turrets at almost max range around my wormhole and spread around the wormhole, so when an astrotrain motorcade busts through only a very small portion of my defenses go down, and thats where the remains rebuilders come in. (
I'll come back and attach a screenshot of my example wormhole defense.)
First two pics shows range of dark matter turret covering tractor and tachyon beams, third pic is my typical wormhole defense. I'd suggest altering turret position a bit so they stay our of the way of the trains as much as possible. Although I wouldn't stack up all your turrets in one place so as to maintain the best coverage that you can with your turrets.
Another option would be to map out the ranges of the astro trains as they fly through your system and build your defenses around that, although I think that might be pretty hard to defend.
Note that some of the Zenith units use dark matter, like the Polarizer (which is deadly to turrets).
Never knew that, infact Polarizers are one of the ships I had the biggest problem with destroying my defenses. Now if I can just figure out how to get them to die...
A version of harvester exo-shields that blocks dark matter would be totally excellent.
I would pay. 
dM
Seconded, but maybe that's because I can't bring myself to use them as it is, maybe I'm just missing the point of the exo-shields.
Also with schizophrenic. That reminds me, I was wondering earlier why that is called schizophrenic. This causes the AI to use multiple types of ships per wave if I read it correctly. To me this sounds more like a smarter general than someone who's schizophrenic. Well then I can say I do send multiple types of ships when I attack the AI so I'm schizophrenic I guess that means that could make it true in a way.
I think its meant to imply craziness and lack of predictability, which seems suitable for what it says it does. (never tried it, does it show
for the incoming wave? like "300
to Place in 2:30")
I turn trains off - it's the only starting option I do this with - way too much micro keeping stuff out of their way or time spent killing them...
I like to have them on as they provide attrition to my backwater defenses aside from that they are a minor annoyance. Although I wish there was an option to disable astrotrains showing up on the galaxy map. I'll see I have a few enemies in one of my safe systems, freak out, go there, realize its only an astrotrain then resume normal play, until the next time I open the galaxy map.
