I guthooked my first fish the other day with a circle hook. Don't know how it happened, but it was a 12/0 Mustad circle buried deep in the craw of a 35" shark. So, I put the shark out of it's misery, dug out the hook, and had shark steaks the next day. It's the only time it's ever happened, and although I know I miss a few fish due to the circle hooks, I'd rather lose them than catch them and end up killing them.
Freshwater, I haven't used them enough to tell. Fish that hit and run like bass, I haven't had luck with circle. However, they work well for fish that grab and swallow, like bream and trout. I think people using trout bait (i.e. Powerbait) should use circles to make for easy catch and release.