My 2 cents on why after 120 hours of gameplay I haven't touched circuit networks yet:
1. It's strange that it appears as some very early tech, but actually you can use it only when you can produce smart chests, and they require steel, and by this time you already have at least belts/inserters production automated. So main use of circuit network as of production buffer limiter is actually unusable until some not-so-early stage, and you already stick to red cell marking in target wooden chests instead of putting red wire and smart inserter. Perhaps if chests would have no red-cell marking possibility, more players would be enforced to use circuit network and love it. Afaik some of that comes in 0.13 with everything becoming circuit-connectable, that's good news if wooden chests are now CN-connectable.
2. By the time you get logistic network, you start caring more about limiting whole reserves in your logistic network instead of caring for just one target crate on circuit network. The reason - when you disassemble something or use logistic trash slots, all those products go to some random storage chest instead of passive provider chest near factory output. So again smart inserter near factory starts operating on logistic network conditions instead of circuit network conditions.
3. I once though where circuit network would be useful in my cases - that was to supply remote outposts with repair packs, turrets, walls, etc... without wasting 50 turrets in dedicated train slot. Circuit network would come in handly, but it would require me to drag red wire through all those poles down to the outpost. And if I have two outposts along that single power line, I'd have to put a green wire. And remember which wire for which outpost. Or reassign turrets to locomotives via some combinator to have it on same red wire and remember that locomotives are actually turrets for outpost B. Or even drag another power line just for the sake of extra signal with correct name on it (Turrets). That appeared too quirky to remember and manage, so I didn't even try
What I would propose regarding circuit networks:
Compared to logistic network they are not that useful. Really. I mean as a separate bus. Why need red/green wires at all while everything in factory is already covered with power poles with copper wire on them - can it be high-frequency information transport medium instead of that wiring? Because in early game you care about some particular crate. Later you care about logistic network, and frequently about REMOTE logistic network. So I'd replace all that wiring with one simple decision:
a) Allow to name chests (via chest interface).
b) Allow to name logistic networks (via roboport interface).
c) Consider everything to be on the same red wire if it's on the same electric network.
d) Red wires may still remain to connect distinct electric networks to be on the same circuit bus.
e) Now - the final part. Allow assembly machines and inserters to have arbitrary formula conditions, e.g. "Outpust1.Walls < 50" or "(Trains+Wagons)*Beacons > 20" written with text. Clicks on icons may substitute names or even embed icons in text editor with autoreplacement.
That will solve combinator compexity problem as well. For when I see screenshots full of combinators that reminds me of blueprints from Unreal Engine where editing shaders was simplified so that artists could do that, and eventually stuff like "A + (B+C)*D" takes the whole screen like a block schema, and becomes even harder to perceive and draw than original formula would be, even for artist, not programmer.
The point here - giving a full programming language is certainly overkill, that's why UE, Maya and other software all have of blueprints to tune materials by non-programmers, but still any non-programmer is capable of writing a formula and actually does that in his head before starting to drag arrows in such editors. That actually was resolved in UE later as "math expression" node which binds inputs and outputs, schemes have become much better since then. And I think this is what Factorio actually needs. The format may be as simple as "ItemType" (signal type) or "ChestName.ItemType" or "LogisticNetworkName.ItemType" - perfectly readable, editable and resolves a lot of things. And everything is on the same bus already through power poles with sort of high-voltage DSL.
Sorry for writing a lot about thing I did not even try to use yet, but that sorta explains why I didn't.
