I use strike indicators all the time (unless i'm using a dryfly with no dropper), whatever the water conditions. However, I change what type of indicator I use (as well as where its placed) based on the conditions of the water. If the water is fast I go with yarn indicators, if its slow, I'll place a small bit of puddy on the line, etc.
I have great success with using indicators, but do not reley on them exclusively when nymphing (about 1/4 of my catches come from "intuitively" yanking on the line when its going through an area that seems like prime trout feeding area).
What I have discovered is that in order to effectively use a strike indicator, you have to take into account how the line is behaving under the water. I set up my indicator based on the desired drift I want, and if I get it right, an indicator can detect even the slightist strikes.
Indicators, like weight, tippet, and fly selection, need to be constantly adjusted to fit the conditions of the water and the fish. I don't think many people realize this, and as a result miss a lot of strikes when using them.