I've tied a few of my go to patterns on circle hooks toward the end of last year and had some luck so far. They make it a little harder to tie if it's a small hook ( 1/0 or less) because of the gap but you get used to wrapping at an angle. I can't swear to them because of the limited body of work so far, but I caught my biggest striper ever on one last year, a 27" bruiser.
Don't know if it was a case of the right place at the right time or a better hook up with the circle hook.
I've been tying on Circle Hooks since last year, and initial results have been positive.
Tied some Gibbs and Ray's and Eelpunts on circles, and used them to catch dead drift and wet fly swing fish. The fish hook themselves on the dead drifts. I think I caught fish I never would have noticed before.
I've been using streamers and deceivers tied on the Mustad C71S circle hook since last spring.
While it's true that when you do take a fish it is squarely hooked in the jaw, easy to be released, the majority of flyrod fish I take are anyway hooked in the jaw (or close too it). The nature of fly fishing makes it such that it's rare to have a gut hooked fish. If anything you get a throat hook and even then it's usually with small fish.
I've taken fish and lost fish with flies on circles. Can't say they did any better or worse than J hooks. I'll continue to test them out but at this time I see no real advantage to flies on circle hooks and some notable disadvantages.