My theory on circle hooks is that they work great when the fish has engulfed the fly and is swimming away from you. Although I get very few deep hookings per year when fast stripping sand eel or pogie patterns (2/0 - 5/0), the crab flies get taken deep 2 or 3 out of ten times. Therefore I've started tying them on circle hooks, and am also considering tying them on regular (verses stainless steel) hooks.
I haven't seen fly-sized circle hooks in stainless, although I'm sure they exist. I've also seen circle hooks with such a wide and open gap that they probably don't serve the purpose. When I first started seeing these for the offshore halibut fishery ten years ago, they looked as curly as a copyright symbol. Lately there are "circle" hooks which look more like "radical beak points". What counts is whether they serve the purpose or not.
There are some flats where crab flies are king and this year I hope to have some first hand observations on the benefits of circle hooks.