Circle hooks are great if you're using bait of some kind. You don't set the hook, you just raise you're rod and put pressure on the fish and the hook will get them in the corner of the jaw almost every time. If you jerk, you'll pull the bait our of the fish's mouth.
I haven't tried them with any kind of artificial bait, I'm thinking they wouldn't work well or we'd see some factory baits with circle hooks.