Every type of exercise should be to an intensity that you can't think about anything else than how your body is responding. That goes for cardio, strength training, aerobic, anerobic intervals, no intervals, slow twitch, fast twitch, whatever. Circuiting training advocates claim time savings, but there really is no saving. They work different systems.
I try to do an hour per day, plus warm-up plus cool-down. I mix up what is in that hour, but the basic principle is that the intensity does not permit me to think about anything else.
