The word "fad" is not always pejorative, but if you use it to describe something that others take seriously, then it comes off as insulting.

Is there a word for something loved by the masses but whose true value is lacking?
Once you have a decently size ground force (~ 6 troopers?), you want to start upgrading them, right? Now ... if you're seeing some concrete threat like Mutas or Tanks, then you'll want a specific upgrade like Blink or Charge.

But ... assuming you haven't found any concrete intel at this point, what do you lay down first, Twilight Council or Forge?

While Forge in itself isn't that expensive, you can't seriously plan to build both Forge and Twilight Council and and have enough minerals / gas left to buy upgrades at both.

One advantage of Twilight is of course enabling High/Dark Templars. So ... how (and when) do you decide which of them to get first?
